The juror, an unidentified older man, began moaning as he listened to an expert testifying in defense of Massachusetts General Hospital surgeons Charles Ferguson and Richard Hodin. Weis claims they botched his care after gastric bypass surgery in June 2002.
The judge immediately ordered the other jurors out of the courtroom, but some saw Ferguson, Hodin and other doctors who were in the courtroom rush to the juror's aid.
An attorney for Weis said it was with "great reluctance" that he ask for the mistrial in the case that was expected to go to the jury Wednesday.
"I cannot think of an instance there would be more reason then when a juror has this kind of incident," attorney Michael Moan said, noting some jurors had seen the doctors attending to the juror.
A lawyer for the doctors said a mistrial would be unfair to the surgeons, who had rearranged their schedules to accommodate Weis, who is in the offseason for football.
Judge Charles Spurlock, however, agreed that a mistrial was needed.
"The integrity of the court is more important than schedules," Spurlock said.
The juror collapsed while listening to the testimony of Dr. David Brooks, an expert who was saying the doctors acted responsibly in their treatment of Weis.
Spectators were ordered out of the courtroom shortly after the man fell ill. He was taken away in an ambulance; his condition and identity were not immediately known.
Weis accuses Ferguson and Hodin of acting negligently by failing to recognize life-threatening internal bleeding and infection after his surgery.
The trial, which featured testimony from Patriots quarterback Tom Brady, was entering its second week.
Spurlock consulted with both sets of lawyers and spoke to the jurors after the man fell ill. One juror said he did not know if he could put the incident out of his mind in his deliberations.
William Dailey Jr., the lawyer for the doctors, said he wanted the trial to go on, saying a mistrial "would be terribly unfair to these doctors."
"They responded the way they were trained to do," he said. "They simply stood up and tried to help."