Monozygotic and same-sex dizygotic twin pairs discordant for schizophrenia were compared with matched control twin pairs in order to disentangle genetic and environmental contributions to variation in hypothalamus volume. A decrease in hypothalamus volume was found in patients or discordant twin pairs compared to healthy controls which could be attributed to the decrease in total brain volume. Higher within-twin pair similarities in monozygotic compared to dizygotic twin pairs, suggests that hypothalamus volume might be partly genetically controlled.