In a case-record study, all first-contact patients with non-affective functional psychosis from a defined area over 20 years were diagnosed according to operational criteria of varying stringency and emphasis, and incidence rates for each set of criteria determined by sex and age at onset; data on premorbid adjustment were also analysed by sex and age at onset. The overall first-contact incidence of non-affective functional psychosis was approximately equal in men and women; however, the ratio of male to female incidence rates rose progressively when RDC (1.2), DSM-III-R (1.3), DSM-III (2.2), and Feighner (2.5) criteria for schizophrenia were applied. Schizophrenia was most common in young males and least common in older males, with females occupying an intermediate position. Schizophrenia in young males, particularly when stringently defined, was especially likely to be associated with single status, poor work and social adjustment, and premorbid personality disorder. The results suggest that schizophrenia syndrome is heterogeneous, and young males are especially prone to a severe neurodevelopmental form of illness associated with premorbid deficits.