OBJECTIVE: This study was undertaken to determine whether the incidence of schizophrenia is equivalent for males and females. METHOD: An attempt was made to identify every first-episode case of psychosis in a large Canadian city over a period of 2 1/2 years. A comprehensive referral network was established that included hospital and community settings where psychotic persons might appear. More than 300 potential subjects were identified, 175 of whom underwent a structured psychiatric interview and were assigned diagnoses according to five different diagnostic systems. RESULTS: The incidence of schizophrenia was two to three times higher among males than among females. Even though the use of different diagnostic systems yielded slightly different risk rates, the elevated risk for males remained consistent. There were no differences between the sexes in the incidence of affective psychosis. In comparison with schizophrenia, the incidence rates for mood disorders with psychotic features were sometimes lower and sometimes higher, depending on the diagnostic system used. CONCLUSIONS: The findings, coupled with reports in the past 10 years from other investigators, challenge the conventional belief that the incidence of schizophrenia is the same for the two sexes.