Tom Burgess was 18 and an infantryman when he saw a young Vietnamese girl walking toward him, a live grenade clutched in her hand. Burgess warned her to halt. When the girl kept coming, the American teen-ager made a quick decision: He shot her.

And there was more.

A combat buddy whose head was shot away. A GI killed because there was no room for him in a bunker. And one murderous 48-hour period, pinned down in a graveyard, when Burgess shot two enemy soldiers, then watched as seven of his comrades were riddled with bullets.

This week defense lawyers asked a federal jury in Alexandria to believe that the traumas of Vietnam filled Burgess with such psychic pain -- with such "survival guilt," in the words of a psychiatrist -- that he was effectively in the grip of mental disease.

It was a compelling need for self-destruction, according to the defense, that led Burgess to sell 5 kilos of cocaine -- more than 11 pounds -- at $58,000 a kilo to a man he knew as Bill in the lobby of an Arlington hotel in May. Bill was an undercover agent of the U.S. Drug Enforcement Agency.

After nearly three hours of deliberation -- and two days of testimony in the first use of a so-called "post-traumatic stress disorder" defense in Northern Virginia -- Burgess was convicted of three cocaine-related counts.

One juror who asked not to be identified said last night that only three members of the panel were undecided at the beginning of deliberations. "We all agreed that Vietnam was a stressful event," the juror said, "but we didn't feel the defendant had lost his powers of decision."

Burgess, 32, of Lake Worth, Fla., a strapping 6 feet 4 with blond, curly hair, winked at his sister and gave a quick smile as the verdict was read, but later grasped attorney John Zwerling in a bear hug and both men left the courtroom in tears.

U.S. District Judge Richard L. Williams set sentencing for Oct. 30. Burgess -- found guilty of conspiracy, possession of cocaine with intent to distribute, and use of a telephone in promoting the drug deal -- faces a maximum of 35 years in prison.

A key defense witness, psychiatrist Stephen M. Sonnenberg of the National Veterans Law Center at American and Howard universities, testified Burgess exhibited most of the symptoms of the disease known popularly as "Vietnam syndrome" -- an ailment made famous in part by the 1970s Vietnam movie "The Deer Hunter."

Burgess, the psychiatrist said, suffers nightmares, flashbacks and powerful thoughts about combat, sleep disturbances, a heightened "startle response" to loud noises, loss of memory and concentration and sudden intensification of his symptoms.

"He got very upset when he saw 'Deer Hunter,' " Sonnenberg said outside the courtroom. Sonnenberg said as many as 300,000 veterans are believed to be possible sufferers from the disorder, which is recognized by the American Psychiatric Association.

Although Burgess often appears normal, the psychiatrist testified, the disease has affected his job performance, led to serious drinking bouts for "self-sedation," destroyed his marriage and led inexorably to last May's abortive drug deal.

Sonnenberg told the jury Burgess once thrust his fist through a windowpane, then started crying, "Medic! My leg," in an apparent flashback to shrapnel wounds that he suffered in Vietnam.

Prosecutor Karen Tandy countered that Burgess was "a creative, intelligent salesman" who concocted much of the story detailed by the defense. A federal drug agent from the agency's Fort Lauderdale office, in the area where the conspiracy began, testified the defendant was "a professional drug dealer."

The prosecution produced its own expert testimony from psychiatrist F. J. Pepper of Alexandria, who told the jury he had examined Burgess and found no sign of the disease as set forth by Sonnenberg.

In her closing argument, Tandy hammered away at defense testimony, stressing that Burgess, in a telephone conversation with undercover agent William D. Little that was played in court during the two-day trial, had boasted he could "quadruple the numbers the amount of cocaine he could supply from South America inside a week."

Prosecutors also put on the stand one of two codefendants, who already have pleaded guilty to reduced charges in the case. The witness, Glenn McCoy of Georgia, said Burgess had told him after the drug arrest, "They may think I'm sane now, but they won't at the trial."

The government produced a series of witnesses, including Barry Barrington, the federal informant who led agent Little to Burgess initially and who testified under a grant of immunity from prosecution, but the defense conceded the facts of the cocaine conspiracy.

Defense attorney Zwerling emphasized instead what he called Burgess' insanity, pleading with the jury, "Don't let Tom succeed in destroying himself by getting himself convicted of this charge."

Burgess sat during much of the trial with his head in his hands, nervously tapping his feet under the defense table for 20 and 30 minutes at a time.

The jury listened in rapt attention late Wednesday afternoon as Sonnenberg, the psychiatrist called by the defense, laid out in detail what he called "the disease of guilt" from which he said Burgess suffers.

The narrative -- often interrupted by strenuous objections from the prosecution about its relevance -- touched on Burgess' early home life. Sonnenberg said Burgess' mother, an alcoholic, frequently abused her son and said Burgess grew up believing his father had died when the son was 3 years old.

Feelings of guilt and rage at the death of his father were a precondition of Burgess' "diminished capacity to deal with Vietnam," Sonnenberg said.

Burgess, who returned home as a sergeant in April 1979, spent the ensuing years in various jobs and engaging in heavy drinking. He married in 1976 and for a time, Sonnenberg testified, there were relatively tranquil moments.

His peace was shattered two years ago when Burgess' mother died. At the funeral, Burgess learned from relatives that his father had died when Burgess was 16, and was an alcoholic who had abandoned his family, stole drugs and spent time in prison for the illegal sale of amphetamines, Sonnenberg said.

Burgess' mental reaction, the psychiatrist contended, was to try to get the painful idea out of his consciousness by identifying with his father and then punishing himself by self-destructive acts. Those acts led to separation from his wife, quitting his job as an insurance salesman, and other events, according to Sonnenberg.

"I don't believe Tom Burgess had any choice about selling drugs in May 1981," Sonnenberg testified.