BackgroundIt has previously been reported that births of individuals who later develop schizophrenia vary by birth year.MethodBirth data were analysed on 34 024 individuals diagnosed with DSM–III–R disorganised, catatonic, and undifferentiated schizophrenia using time series analysis.ResultsMinor yearly fluctuations were observed but did not achieve statistical significance. Thus earlier findings reported in the literature could not be replicated using a different statistical approach.ConclusionThe findings do not support theories which assume major yearly fluctuations in the births of individuals with schizophrenia. Such theories include the perinatal effects of influenza, temperature variation, and the effects of severe weather.