To investigate the possibility that lateral cerebral ventricular size may be under genetic control, we compared the computed tomography (CT) scans of 17 healthy siblings from 7 normal sibships. The CT scans of 10 chronic schizophrenic patients and 12 of their nonschizophrenic siblings were also compared. A trend was found for a correlation of ventricular size between siblings in the healthy sibships (ICC = 0.25, p = 0.1) but not in the schizophrenic sibships (ICC = -0.05). In each sibship the schizophrenic patient had the largest ventricles; in seven cases they exceeded the normal range. Although the discordant siblings were all well within the normal range, their ventricles were larger (p = 0.001) than those of the controls. The findings suggest a genetic component to ventricular size in healthy individuals and that CT findings in schizophrenics are not coincidental familial traits but markers of the illness. The implications of the findings in the discordant siblings are discussed.