Few studies have examined the effects of age and gender on the utility of diagnostic computed tomography (CT) for appendicitis. We retrospectively studied all adult patients undergoing appendectomy from January 2000 through December 2002 (633 patients). Patients 15-30 years old ("younger") were compared to patients >30 ("older") and further subdivided by gender. CT accuracy and the influence of CT on negative appendectomy (NA) rates for each group were evaluated. CT was associated with a lower NA rate in older patients (23% vs 8%, P = 0.004) but had no effect on NA rate in younger patients (26% vs 20%, P = 0.2). Appendiceal CT had greater sensitivity, negative predictive value, and accuracy in older than in younger patients. When subdivided by gender, CT was associated with lower NA rates for all female patients but had no effect in either age group of men. Although CT may be more accurate in patients older than 30 years, it has the greatest benefit in female patients, where it is associated with lower NA rates in all adult patients. CT should be strongly considered for all female patients before operation for suspected appendicitis. Further study is needed to determine which, if any, male patients benefit from preoperative CT.