The hypothesis that the gene for schizophrenia is located in the pseudoautosomal region of the sex chromosomes predicts that same-sex concordance will occur in paternally rather than maternally derived pairs. In 120 families that included at least one sibling pair with schizophrenia, affected members were significantly more likely to be of the same sex when there was a history of illness on the paternal than on the maternal side, the difference remaining significant when parent of origin was assessed by three different methods. The finding is as predicted by the pseudoautosomal hypothesis: therefore a search for the gene should be focused on this small (three megabase) region of the genome. The ratio of same to mixed sex pairs in paternally-derived cases (approximately 3:1) suggests the gene is located in the centromeric one-third of the pseudoautosomal region.