Because the social-interpersonal symptoms of schizotypal personality disorder have been more useful than the cognitive-perceptual in identifying the biological relatives of schizophrenics, it has been recommended that the schizotypal DSM-III-R (or DSM-IV) criteria be revised to emphasize the social-interpersonal items. This study determined whether the social-interpersonal items would be more efficient than the cognitive-perceptual in diagnosing schizotypal personality disorder in patients presenting to a clinical setting. In contrast to studies conducted in nonclinical samples, we found that the cognitive-perceptual items were equally and at times more important than the social-interpersonal items to the diagnosis of patients with schizotypal personality disorder. We demonstrate that the value of an item depends in part on the particular differential diagnosis at issue.