Using five methods of validation the authors evaluated six patients satisfying DSM-III criteria for schizophreniform disorder. These patients did not differ importantly from patients with affective disorders but differed markedly from schizophrenic patients in past psychiatric history, family history, acute treatment response, short-term course, and dexamethasone suppression test results. The findings suggest that schizophreniform disorder, as defined by DSM-III, may not be a valid entity separate from affective disorder and that acuteness of onset, even in the absence of affective symptoms, implies the presence of affective disorder. The authors suggest that a diagnosis of schizophreniform disorder should not exclude patients from treatment with lithium and antidepressants.