Introduction:A family history of appendicitis has been reported to increase the likelihood of the diagnosis in children and in a retrospective study of adults. We compare positive family history with the diagnosis of acute appendicitis in a prospective sample of adults.Methods:We conducted a prospective observational study of a convenience sample of 428 patients. We compared patients with surgically proven appendicitis to a group without appendicitis. The latter were further grouped by their presenting symptoms: those presenting with a chief complaint of abdominal pain and those with other chief complaints. Participants answered questions regarding their family history of appendicitis. Family history was then compared for the appendicitis group versus the nonappendicitis group as a whole, and then versus the subgroup of patients without appendicitis but with abdominal pain. The primary analysis was a χ2 test of proportions and the calculation of odds ratio (OR) for the relationship between final diagnosis of appendicitis and family history.Results:Of 428 patients enrolled, 116 had appendicitis. Of those with other diagnoses, 158 had abdominal pain and 154 had other complaints. Of all patients with appendicitis, 37.9% (confidence interval [Cl] = 29.1–46.8) had positive family history. Of those without appendicitis, 23.7% (Cl = 19.0–28.4) had positive family history. In the subgroup without appendicitis but with abdominal pain, 25.9% (Cl = 19.1–32.8) had positive family history. Both comparisons were significant (P = 0.003; OR = 1.97; 95% Cl = 1.2–3.1; and P=0.034; OR = 1.74; 95% Cl = 1.04–2.9, respectively). By multivariate logistic regression analysis across the full sample, family history was a significant independent predictor (P = 0.011; OR = 1.883) of appendicitis.Conclusion:Adults presenting to the emergency department with a known family history of appendicitis are more likely to have this disease than those without.