This paper focuses on the classification of mood disorders. Data are reported from 227 outpatients who met DSM-III-R criteria for Major Depressive Episode. Each patient was evaluated by the Semistructured Interview for Depression (SID), which was developed and organized according to a decision tree model. The SID was used both to identify the sample with major depressive disorders and, then, to subclassify them into five subtypes. Three bipolar types (I, II, III) and two unipolar types (recurrent and single episode) were distinguished, and comparisons among the subtypes are presented. Therapeutic implications of the classification are discussed in relation to recent advances in targeting short and long-term treatments for specific subtypes.