BACKGROUND: Previously reported associations between advancing paternal age and schizophrenia could be due to an increase in paternal germ cell mutations or be confounded by heritable personality traits associated with schizophrenia that result in delayed parenthood. AIMS: To investigate this association while adjusting for personality traits related to poor social integration in the subjects. METHOD: A cohort of 50,087 adolescent males was followed up by record linkage to determine hospital admissions for schizophrenia between 1970 and 1996. RESULTS: Advancing paternal age was associated with an increased risk of developing schizophrenia in a 'dose-dependent' manner. The adjusted odds ratio for each 10-year increase in paternal age was 1.3 (95% CI 1.0-1.5; P=0.015). CONCLUSIONS: Advancing paternal age is an independent risk factor for schizophrenia. Adjusting for social integration in subjects made little difference to this association, consistent with the hypothesis that advancing paternal age may increase liability to schizophrenia owing to accumulating germ cell mutations.