Computed tomography (CT) is used commonly to "rule-out" disease. In 123 consecutive patients with psychiatric diseases in three institutions, 105 CT scans (85.4%) interpreted as normal or normal except for atrophy did not influence management, only reassured clinicians and patients. Incidental and false positive findings were more numerous than true positive findings. All six true positive findings occurred in patients with focal findings on neurologic examination; four led to management changes. The lower 954% confidence limit of the cost of case finding by CT only was estimated to be $2931 for all patients; $603 for patients with focal findings; and at least $7083 for patients without focal findings. Computed tomography is a sensitive diagnostic method that may detect more false and incidental positive findings when used somewhat unselectively to "rule-out" disease. In these patients, a examination suggesting structural intracranial abnormalities would have been less expensive and more effective.