Families with more than one individual of the same generation with the diagnosis of schizophrenia were recruited for studies. Brain lateral ventricular size was quantified in 26 schizophrenic subjects from 12 unrelated families, their available well siblings (N = 10), and 20 nonpsychotic controls. Lateral ventricular size was significantly greater in the schizophrenics than in their well siblings and controls. In addition, an analysis of variance of the data showed a significant familial component to ventricular size. Although histories of head injury and birth complications were also associated with ventricular size, these were not sufficient to explain both the familial aspect of ventricular size and the association of greater ventricular size with schizophrenia within these families.