The trial of former WorldCom Inc. chairman Bernard J. Ebbers gets underway this week, beginning a new round of courtroom confrontations over events that helped write the emphatic end to last decade's investment boom.

WorldCom and Enron Corp. fell apart and HealthSouth Corp.'s stock plunged after revelations of fraud that had hidden fundamental weaknesses. Pensions, jobs and billions of dollars in shareholder equity were lost in the wreckage.

Now the executives who led those companies are fighting to stay out of prison. Opening statements in the trial of former HealthSouth chairman Richard M. Scrushy begin next week, and Enron's former chairman, Kenneth L. Lay, is expected to face trial later this year, though no date has been set.

Both prosecutors and defense attorneys will enter the courtrooms in those and other corporate trials armed with lessons learned in last year's round of corporate trials, which led to the convictions of Martha Stewart and Frank P. Quattrone -- but also to a hung jury in the trial of former Tyco International Ltd. executives and a handful of acquittals.

The government's lawyers, seeking to hold the men at the top accountable in the cases of Ebbers, Lay and Scrushy, will be aided by the cooperation of lower-level employees who have already pleaded guilty.

The government's aim in seeking convictions of top executives is deterrence, outside analysts said. "The point of these trials is to signal to other CEOs that, while the corporate governance system is broken and the chances of detection are depressingly slim, the consequences of discovery are so dire that it's irrational to engage in fraud," said Jonathan R. Macey, a Yale University law professor.

The three cases will focus on one crucial question: How much did these high-profile executives know about the fraud that brought their companies low? When times were good, these men took credit for their firms' successes, but now Ebbers, Lay and Scrushy are claiming they were out of touch and misled by corrupt underlings who used off-the-books partnerships and accounting tricks to inflate profit.

Defense lawyers, therefore, will try to persuade jurors to distinguish these high-profile businessmen from the bad acts of the companies they led.

"These cases terrify me because of the huge numbers, impossibly complicated accounting issues and all the negative publicity," said Reid H. Weingarten, Ebbers's attorney. "I remain hopeful the jury will do what juries are supposed to do: focus only on the evidence and follow the law. . . . If that happens, Bernie Ebbers will be acquitted."

One of this year's trials is a rerun. Former Tyco chairman L. Dennis Kozlowski and his co-defendant, former Tyco chief financial officer Mark H. Swartz, face grand larceny charges. Prosecutors say they stole $600 million from the company; the defense argues that the two men had the authority to grant themselves bonuses and forgive company loans. An earlier trial ended in a mistrial after a juror's name was publicized.

Jury selection starts in Manhattan state court today for Kozlowski and Swartz, whose attorneys did not return phone calls seeking comment.

Tomorrow, the scene will shift down the street to Manhattan's federal court, where another pool of potential jurors will begin filling out questionnaires for Ebbers's trial. Opening statements are expected Jan. 25 for Scrushy's trial in Birmingham. Lay's trial is expected to begin in Houston later this year, though no trial date has been set and Lay has requested another venue.

The cases open at a time of confusion in the federal courts. A Supreme Court ruling last week partially invalidated the federal sentencing guidelines and complicated the government's goal of multiyear prison sentences. Judges will have far more discretion in sentencing and may be more open to pleas for leniency.

Prosecutors in all three cases declined to comment publicly on their trial plans, but outside legal analysts said the government's key challenge will be to prove that the chief executives knew their actions and those of their underlings were illegal. The government's best evidence will probably come from high-ranking cooperating witnesses who can shed light on what the chief executives knew and were thinking when they approved actions that prosecutors now say were illegal.

Ebbers refused to use e-mail professionally, so prosecutors in the WorldCom case will probably have to rely heavily on the testimony of the company's former chief financial officer Scott D. Sullivan, who pleaded guilty last March to conspiracy and securities fraud. He is expected to tie Ebbers to the firm's decisions to improperly report operating expenses as capital expenses and to make unannounced changes in the way it reported revenue to make it look as if WorldCom were meeting Wall Street analysts' expectations when it was not. The telecommunications giant filed for bankruptcy protection after revealing a record $11 billion accounting fraud and now does business as Ashburn-based MCI Inc.

"One of the great advantages of having an insider testify is that they become a tutor to the jury as to what was going on," said Daniel C. Richman, a Fordham University law professor and former prosecutor.

Ebbers's defense attorneys, meanwhile, will probably argue that their client, a former milkman and high school coach, left accounting decisions to Sullivan and that the finance chief has turned on his former boss to save himself from a long prison term. Some WorldCom officials say there has been bad blood between the two men for years.

In Scrushy's case, his legal team has a legion of government witnesses to attack. Birmingham's U.S. Attorney Alice H. Martin has secured guilty pleas from more than a dozen former HealthSouth employees, including five former finance chiefs. Now she is seeking to convict Scrushy on 58 criminal counts, including securities fraud, money laundering and violating provisions of a 2002 corporate accountability law that requires chief executives to vouch for the accuracy of their companies' financial statements.

Scrushy vigorously denies the charges and says his onetime lieutenants masterminded the $2.7 billion fraud and hid the scheme from him. His lawyers are raising questions about the witnesses' medical histories, marital fidelity and past alcohol and drug use. The defense team also has asked the judge for access to witnesses' employment records.

James D. Wareham, a corporate defense lawyer who has been following the HealthSouth investigation, said Scrushy's lawyers have a difficult task ahead of them.

"It's easier to discredit one or two felons as demonstrable liars," said Wareham, of the law firm Paul, Hastings, Janofsky & Walker LLP. "When you have five individuals, that makes it a lot harder to carry the day, because all the jury has to do is believe one guy."

Charles Russell, a spokesman for Scrushy, declined to comment on the case.

Outside analysts said both Ebbers and Scrushy will feel pressure to take the witness stand to make the case personally that they relied on advice from lawyers and accountants -- or that subordinates kept them in the dark about the accounting schemes.

"In most of these cases you're talking about a CEO who is not an accountant," said Michael N. Levy, a defense lawyer at McKee Nelson LLP in Washington. "Prosecutors have got to prove that the CEO knew the accounting decision was wrong at the time but went ahead with it anyway."

That issue, apparently, helped deadlock the jury in the Cendant Corp. fraud case earlier this month. On Jan. 4, after 33 days of deliberation, the jury found former vice chairman E. Kirk Shelton guilty of conspiracy and securities fraud charges. Jurors were unable to reach a unanimous verdict on the role played by former chairman Walter A. Forbes.

Cendant insiders, including former officials in the accounting and finance units, pleaded guilty and testified against both men during the trial. But documents and testimony fingered Shelton as more directly involved in more than $500 million worth of revenue inflation that occurred before their company, CUC International Inc., merged with HFS Inc. to form Cendant in 1997.

New Jersey's U.S. Attorney Christopher J. Christie is considering whether to retry Forbes on the 16 charges, he said in a prepared statement.

John C. Coffee Jr., a Columbia University law professor, said Forbes's defense, in which he testified that he only halfheartedly reviewed financial reports and was deceived by key insiders, could give a measure of hope to high-profile executives awaiting trial later this year. One such former executive is Lay, who faces fraud charges related to statements he made before the Houston energy trader collapsed in 2001. A federal judge has yet to set a trial date for Lay and former chief executive Jeffrey K. Skilling, who in effect led Enron through the late 1990s, when prosecutors say the fraud occurred.

"I would have more optimism about Mr. Lay's chances, because he was sort of a distant, hands-off manager who had resigned and come back," Coffee said.

But he added that some corporate managers may have more trouble convincing juries that they were out of the loop. "Anyone who watches Bernie Ebbers knows he was a very strong person, and Scott Sullivan was his lackey," Coffee said.

Convicting a former chief executive becomes even more challenging when the government is unable to secure cooperation from central players just below the chief executive's level. At the first Tyco trial, Swartz, the former finance chief, not only did not testify against Kozlowski but actually took the stand and testified over several days that the company's board had given Kozlowski and him the authority to pay themselves bonuses.

The case ended in mistrial after media reports revealed the name of a juror holding out for acquittal. That result and the mistrial last month in the fraud trial of former Westar Energy Inc. chief executive David C. Wittig suggest that prosecutors may face difficulty proving criminal intent -- a crucial requirement in white-collar crime cases -- unless they can point to specific actions that misled investors.

"It's not enough to prove these guys got billions. Getting billions is not a crime," said Steven M. Cohen, a former federal prosecutor who is now a defense attorney. "Prosecutors have to prove to the jury that they did something illegal to get those billions. The defense attorneys can take some comfort in that."

Still, most legal observers say the defense lawyers face an uphill battle. With the exceptions of former Tyco general counsel Mark A. Belnick and a lower-level executive at Adelphia Communications Corp., "it's hard to remember a major fraud case that went to a jury trial and led to an acquittal," said Robert Weisberg, a Stanford University law professor. "I'm betting on the government."