Several recent studies of psychiatric patients have relied upon magnetic resonance imaging (MRI) to demonstrate features of cerebral anatomy in the midsagittal plane. Methodologies have varied somewhat in relation to thickness and position of planes of view. Due to concerns over the effects of slice thickness and position on measurements, the authors used a multislice thin plane method to assess these effects in 143 individuals (34 controls, 58 schizophrenics, and 51 with bipolar or schizoaffective disorder). Substantial variance in area measurements attributable to slice position emerged, especially in ventricular, cerebral and cerebellar area. Of greater importance would be the demonstrated interaction of position with diagnosis and sex in measures of several regions. The implications of these findings for MRI studies are discussed.