To identify differences between correctly diagnosed appendicitis and misdiagnosed cases that resulted in litigation between 1982 and 1989 retrospective review of malpractice claims was conducted. A total of emergency department (ED) charts at the time of the initial ED visit were reviewed and compared with 66 concurrent controls. Missed cases appeared less acutely ill, had fewer complaints of right lower quadrant pain, received fewer rectal examinations, received intramuscular (IM) narcotic pain medication for undiagnosed abdominal pain or symptoms, and more often received an ED discharge diagnosis of gastroenteritis. Misdiagnosed patients had a 91% incidence of ruptured appendix, more extensive surgical procedures, and more postoperative complications. Data were analyzed using the Pearson's chi 2 Test, Mann-Whitney U Test, and stepwise discriminant analysis. Significance was defined as P < or = .05. Misdiagnosis of acute appendicitis is more likely to occur with patients who present atypically, are not thoroughly examined (as indexed by documentation of a rectal examination), are given IM narcotic pain medication and then discharged from the ED, are diagnosed as having gastroenteritis (despite the absence of the typical diagnostic criteria), and with patients who do not receive appropriate discharge or follow-up instructions.