OBJECTIVE: The aim of the present study was to analyse first incidence of psychotic disorders in the Lundby population during a 50 year period by comparing male and female age at onset, overall incidence rates and age-specific incidence rates. METHOD: The Lundby Study is a prospective study of the mental health of a complete community population (n = 3563), which was followed from 1947 to 1997. Data from interviews, registers, case files and key informants were accumulated via four waves of field work (1947 1957, 1972 and 1997). Mean and median age at onset, and overall and age-specific incidence rates, for the first episodes of major groups of psychotic disorders according to the DSM-IV were calculated (the major groups were: any psychotic disorder, psychotic disorder due to a general medical condition, substance-induced psychotic disorder, non-affective psychotic disorder, schizophrenia, other non-affective psychotic disorder and affective psychotic disorder). Male-female differences in mean ages at onset and overall incidence rates were tested. Male-female differences in incidence by age patterns were described. RESULTS: The overall 50 year incidence rate in male subjects was higher than in female subjects for substance-induced psychotic disorder, but for the other disorders the overall rates did not differ significantly between the sexes. The male mean age at onset was lower than that for female subjects for any psychotic disorder, psychotic disorder due to a general medical condition, non-affective psychotic disorder and schizophrenia. Male and female subjects had different incidences by age patterns for any psychotic disorder, non-affective psychotic disorder, schizophrenia and other non-affective psychotic disorder, with a male preponderance among early-onset cases, and a female preponderance among late-onset cases. CONCLUSION: The differences in incidence between the sexes in this 50 year follow up may indicate psychotic disorder-delaying mechanisms in female subjects, or different aetiologies of psychosis in male and female subjects.