Purpose: The purpose of this study was to ascertain whether a physician's use or non-use of a decision aid influenced jurors' decisions in a malpractice trial. Method: A DVD containing one of eight versions of a highly realistic malpractice trial was sent to a national sample of 1000 jury-eligible adults. The eight versions varied in whether (a) the physician did or did not use a decision aid (Alvarado Score) in diagnosing possible appendicitis, (b) whether the physician heeded or defied the aid, and (c) whether the patient's presenting symptoms would have warranted a score of 4 or 7 on the Alvarado Score, the lower score not being compatible with a diagnosis of appendicitis and the higher score indicating that appendicitis was probable. The same adverse outcome occurred in all 8 versions. Mock jurors indicated whether the physician did or did not meet the standard of care, and if he did not, what level of punishment he deserved, which was assessed on a 10-point Likert scale. The former dependent variable was analyzed with a logistic regression, the latter with an analysis of variance. 655 mock jurors returned usable data. Results: On the dependent variable pertaining to met/didn't meet the standard of care, the triple interaction was significant (p < .01). Mock jurors were significantly more likely to deem a physician not to have met the standard of care if the physician used a decision aid, the patient presented with a high Alvarado Score, but the physician ignored the aid's recommendation. Among the 249 mock jurors who deemed the physician not to have met the standard of care, the significant (p < .02) aid usage x heed/defy interaction was caused by harsh punishment meted out to physicians who used an aid but defied it. Conclusions: Physicians who use a decision aid would be well advised to heed its recommendation. If the aid recommends an affirmative course of action, physicians who defy the aid are significantly more vulnerable to malpractice liability verdicts should an adverse outcome occur. Whether the aid's recommendation is positive (“do X”) or negative (“don't do X”), if the jurors deem the physician liable, they are more punitive toward physicians who have defied the aid.