In an attempt to improve the classification of Bipolar II disorders, we have examined a consecutive series of 687 primary major depressives: 5.1% gave a past history of mania (Bipolar I), 13.7% met our operational criteria for hypomania (Bipolar II), and the remaining 81.2% were provisionally categorized as 'unipolar.' Although Bipolar II was in some respects intermediate between Bipolar I and Unipolar, gender, familial bipolar history, age at onset and course characteristics generally supported its closer kinship to bipolar illness. Seventy one of the unipolars (10.3% of the total series) further met our operational criteria for hyperthymic temperament (U-HT), leaving behind a purer unipolar group of 487 major depressives. With respect to the proportion having male gender and bipolar family history, U-HT was similar to Bipolar I and II, and all three differed significantly from pure unipolar; as for age at onset, number of episodes and related indices of course, BI and BII were similar, and U-HT was closer to pure unipolar. These findings suggest that major depressive episodes arising from a hyperthymic temperament (constituting 12.4% of the 'unipolar' universe by conventional definition) are 'genotypically' closer to Bipolar II defined by hypomania, and course-wise similar to other unipolars.