Eleven identical (monozygotic) twin pairs discordant for schizophrenia and 18 unselected control monozygotic twin pairs received a computed tomographic scan. Brain absorption density was determined on quadrants at five slice levels using a fully automatic program that eliminated cerebrospinal fluid spaces from analysis. There was no difference in brain density among schizophrenics, co-twins, and controls. There was a significant difference in right vs left hemisphere asymmetry of density across diagnostic groups. Overall, the left hemisphere was less dense than the right in the schizophrenics, while the reverse was found for the co-twins and controls. These results support the hypothesis of left hemisphere dysfunction in schizophrenia and suggest that it is an environmentally acquired, rather than genetic trait.