Background. There have been numerous reports of seasonal trends in psychotic illnesses. In schizophrenia, seasonal trends in incidence have been shown to be especially apparent in first-episode cases. Most previous research has used date of admission as a proxy for date of incidence of disorder; we present results of an investigation into seasonal trends in dates of onset of symptoms in a group of 295 first-episode cases of schizophrenia and schizophreniform disorder.

Method. Data were analysed using statistical methods appropriate for detecting seasonal trends in pooled data over 6 years.

Results. Only male cases of schizophrenia and schizophreniform disorder showed a significant seasonal distribution to dates of onset of symptoms, with a peak in August (winter).

Conclusion. True seasonal patterns are present in schizophrenia incidence, but their consistency with other published studies and wider significance, is difficult to ascertain because of different methods used in dating incidence of disorder.