The season of birth and season of hospital admission of 50 bipolar I and 42 bipolar II female patients were analyzed. Significant differences were found in the seasonal distribution of births in the two subgroups. The birth dates of most bipolar I patients showed a tendency to peak during spring and autumn, while bipolar II patients were born mostly in summer and winter. Bipolar I patients were hospitalized for mania mainly in spring and autumn, and for depression mainly in summer and winter. Examination of the seasonal variation of hospital admission for depression also revealed significant difference between bipolar I and bipolar II patients. The results support the validity of the bipolar I-bipolar II distinction and are in agreement with earlier findings.