Twenty-six patients with RDC bipolar disorder were compared with a previously reported group of 48 RDC schizophrenics and 34 healthy controls, using volumetric MRI measurements of cerebral, cortical and sulcal volumes. The bipolar group appeared no different from the controls, and both of these groups had significantly larger cerebral and cortical volumes than the schizophrenics. Our previous report of a significantly reduced cortical volume in the schizophrenic group, with a corresponding increase in the volume of sulcal fluid is, therefore, not a generalized feature of psychotic illness but may be more specific to schizophrenia.