We report on the utility of a new instrument to identify subtypes of major depressive episodes with special reference to pseudo-unipolar conditions. By incorporating reliable measures of depressive and hyperthymic temperamental characteristics in subtype definitions, we achieve the sharpest possible demarcation between unipolar and bipolar disorders. The new procedures also reveal that 1 out of 3 primary depressives in a consecutive series of 405 patients belong to the bipolar spectrum. Furthermore, among bipolars, bipolar II disorder (redefined as major depressions with hypomania or hyperthymic temperament) represents the most common variant. We discuss the nosologic, therapeutic, methodologic and theoretical implications of these considerations on the unipolar-bipolar dichotomy. Given that major depression emerges as the final common clinical expression of a heterogeneous group of disorders, it underscores the importance of focusing on temperament and course of illness in subclassification efforts such as attempted here.