To assess the risks of adverse outcomes after appendectomy incidental to cholecystectomy among elderly Medicare beneficiaries, 8,936 persons undergoing cholecystectomy with incidental appendectomy and 44,461 persons undergoing cholecystectomy without incidental appendectomy were studied. Controlling for age, race, gender and co-morbidity status, the risk for wound infection in persons with incidental appendectomy was 83 percent higher than in persons without incidental appendectomy (95 percent confidence interval, 1.53 to 2.18). The risks for having other adverse outcomes, including other infections, extensive intrahospital complications and mortality rate at 30 days, were also higher for the former group, although these differences were not statistically significant. In addition, the demographic characteristics and health status of persons undergoing cholecystectomy with incidental appendectomy with persons undergoing cholecystectomy only were compared. Males, persons of younger ages, of white race or with no co-morbid conditions, were significantly more likely to undergo cholecystectomy with incidental appendectomy. Variables to control for differences in the demographic characteristics and health status between persons receiving and not receiving incidental appendectomy were included in the regression models for adverse outcomes. However, these models may not completely control for differences between the two groups. As a result, the actual relationship between incidental appendectomy and adverse outcomes may be underestimated. The preventive effect of incidental appendectomy on morbidity and mortality rates from future instances of appendicitis was assessed by determining the remaining lifetime risk for acute appendicitis. For persons 65 to 69 years of age, 115 incidental appendectomies would be required to prevent one future instance of appendicitis and 4,472 incidental appendectomies would be needed to prevent a single future death from acute appendicitis. Because incidental appendectomy increases the risk for wound infection among persons undergoing cholecystectomy and because the lifetime risk for acute appendicitis is relatively low for persons of this age group, surgeons should carefully consider the risks and benefits of incidental appendectomy in the elderly.