To investigate a possible association of midline cerebral malformations with psychotic disorders, MRI and CT scans were blindly evaluated for 52 patients with schizophrenia, 9 with schizoaffective disease, and 79 consecutive nonpsychotic control subjects. Midline abnormalities were present in 10 of 61 patients (16.4%) versus 4 of 79 control subjects (5.1%; P < 0.05, chi-square). Of 52 schizophrenic patients, 8 had abnormalities of the septum pellucidum (SP): 5 had cavum vergae (CaV), 2 had cavum septum pellucidum (CaSP), and 1 had agenesis of the corpus callosum and SP. Of 9 schizoaffective patients, 2 had SP abnormalities: 1 CaV and 1 CaSP. Abnormalities of the SP, especially CaV, were significantly more frequent in women than in men (P < 0.02, chi-square).