As one of the country's preeminent appellate lawyers, John G. Roberts Jr. advanced the interests of major corporate clients. Rules that govern conflicts of interest would allow him to hear Supreme Court cases involving those same companies on different issues.

In 13 years of private practice at the District-based law firm Hogan & Hartson LLP, Roberts built his reputation by arguing before the high court on behalf of Digital Equipment Corp., Peabody Coal Co. and Toyota Motor Corp., among others.

Under federal ethics rules, Roberts is barred from hearing only specific cases that he handled at the law firm or other cases that Hogan & Hartson took on during his tenure. Roberts left the firm in 2003 to become a federal appeals court judge in the District. President Bush nominated him Tuesday to fill the Supreme Court seat of retiring Justice Sandra Day O'Connor.

Roberts, 50, could adjudicate disputes involving former clients unless he had worked on the specific issue in question while he was in private practice or in his two years on the appeals court in the District, experts said. "The rules are pretty clear," said Stephen Gillers, a New York University law professor and author of a legal ethics book.

Roberts's former law firm declined to provide a complete list of his former clients, citing a policy that protects client confidentiality.

His years in private practice left him financially comfortable, according to financial disclosure forms. Roberts and his wife, Jane Sullivan Roberts, had a net worth of more than $4 million in 2003, including a home in Bethesda and a partial share of a cottage in Limerick, Ireland, according to a financial disclosure form filed for that year.

Roberts would have to remove himself from any case that involves companies in which he or his wife or young children own stock, according to ethics rules for the high court.

That means -- at least according to his latest financial disclosures, which covered 2004 -- he might have to bow out of many cases that involve some of the country's largest high-tech and pharmaceutical firms.

The report indicates that Roberts owned $100,001 to $250,000 worth of stock in Dell Inc., Microsoft Corp., Time Warner Inc. and XM Satellite Radio Holdings Inc. He also held $50,001 to $100,000 of Intel Corp. and Texas Instruments Inc. Roberts owned $15,001 to $50,000 in Becton Dickinson and Co., Cisco Systems Inc., Citigroup Inc., Walt Disney Co., Freddie Mac, Hewlett-Packard Co., Nokia Corp., Pfizer Inc., and Washington Real Estate Investment Trust.

Roberts also owns shares in several mutual funds including ones managed by Fidelity Investments and Janus Capital Group Inc. Mutual fund holdings are exempt from the recusal rule because shareholders do not make day-to-day managerial decisions about those investments.

Roberts's wife is a lawyer who represents technology clients in the District office of Pillsbury Winthrop Shaw Pittman LLP. According to the law firm's Web site, Jane Roberts has represented clients involved with global satellite systems, software licenses and professional service deals.

Seven high-court justices laid out their own conflict-of-interest policy regarding spouses and children who practice law in an unusual news release in November 1993. The judges concluded they could hear cases in which their relatives' law firms played a role, so long as their family members did not take part in the specific case in front of the court.

They reasoned that multiple law firms often take part in a single case and that parties could try to manipulate the process by hiring law firms where the relatives of justices worked to prevent a particular justice from ruling.

"Even one unnecessary recusal impairs the functioning of the Court," the judges added.

The Supreme Court has nine members. If one judge recuses himself and the case splits evenly 4 to 4, as happens on rare occasions, the lower-court ruling stands.

Legal experts said it is not unusual for Supreme Court justices to step aside from cases. Justices are not required to provide a public reason for their recusal, but most result from the stock ownership rules or instances in which jurists expressed a public opinion on a case.

Business groups had called on the president to select a candidate familiar with -- if not sympathetic to -- their interests, and they appear to have one in Roberts. Justice Lewis F. Powell Jr., who resigned in 1987, is the last Supreme Court jurist to have represented a substantial number of corporate clients in private practice. Roberts is particularly well liked by trade groups because of his winning representation of Toyota in a key 2002 case that limited workers' use of the Americans with Disabilities Act.

Researcher Lucy Shackelford contributed to this report.