To determine whether abnormalities in brain morphology are present at the onset of illness, patients with schizophrenia, schizophreniform and bipolar disorders, and major depression who were experiencing their first episodes of psychosis were compared with normal and medical control subjects. The schizophrenic patients had larger third ventricles but not larger lateral ventricles or cortical sulci than the normal subjects. The other psychotic patients did not differ from the normal group on these measures. A different pattern of results emerged when the medical patients were used for comparison, indicating that the choice of control group can influence the findings of computerized tomography studies.