An evaluation was made of 1030 computed tomography (CT) scans performed on cancer patients. Seventy percent of the scans taken were of the pelvis, abdomen, or thorax. Both referring clinicians and radiologists rated the diagnostic and management efficacy of each scan using a simple scale of values. Over 50% of the scans provided unique diagnostic information: 39% resulted in a change in diagnosis or in known extent of disease; and 14% led to a change in patient management. The fraction of abnormal results was not a reliable measure of efficacy. Specific groups of patients were identified, which demonstrated that both diagnostic and management efficacy should be measured in assessing the value of CT as a diagnostic modality.