BackgroundIn patients with bipolar disorder, admissions for manic and depressive episodes frequently display a seasonal pattern. We examined this variation and compared the patterns with the seasonal admission rates for schizophrenia.MethodPatients with bipolar disorder or schizophrenia were identified from the Finnish Hospital Discharge Register of in-patient admissions to all psychiatric hospitals during the years 1969–91. They were included in the analysis if the first admission had occurred before 30 years of age. A total of 295 bipolar patients were found and a sample of 295 schizophrenic patients was randomly selected for comparison.ResultsThere was no seasonal variation among all hospital admissions for bipolar disorder or schizophrenia However, the first admission for a depressive compared with a manic episode of bipolar disorder occurred significantly more often in the autumn (33% v. 21% respectively). The peak difference occurred during the week after the autumnal equinox in September.ConclusionOur findings suggest that there is no seasonal variation in bipolar disorder, although in some patients the clinical course might be influenced by the autumn, as far as the likelihood of a first admission for depression is concerned.