Helical abdominal computed tomography (HCT) is a common test in the evaluation of patients with presumed appendicitis. Studies have demonstrated HCT to have high sensitivity, specificity, and positive and negative predictive value. Despite this, there has not been consistent demonstration that HCT has had beneficial effect on patient outcome. The objective of this study was to assess the impact of HCT on patient outcome as measured by the rate of negative appendicitis and perforated appendicitis. Patients were identified from a pathology department database that included all patients taken to the operating room with a pre-operative diagnosis of appendicitis. Pathologic specimen analysis was used to determine the presence of appendicitis and perforation. Two periods were studied: Period A, a 4-year interval before the arrival of HCT; and Period B, a 3-year period several years after the incorporation of HCT into the evaluation of suspected appendicitis. Primary outcome measures were the rates of negative appendicitis and perforated appendicitis. During Period A, 316 patients were identified; 12% had conventional computed tomography, none had HCT. The negative appendicitis rate was 15.5%; the perforated appendicitis rate was 11.6%. During Period B, 477 patients were identified; 81.5% had HCT. The negative appendicitis rate was 7.9%; the perforated appendicitis rate was 14.4%. The difference in negative appendicitis rates was 7.6% (3.0%, 12.4%), and in perforated appendicitis it was -2.8% (95% CI -8.0%, 2.1%). At the study institution, there was a 48% decrease in the rate of negative appendicitis encountered in association with the common use of HCT.