The bipolarII disorder (BP-II) was initially diagnosed by Dunner et al.(1976) from the bipolarI disorder, and the distinction of the two types was then officially standardized by DSM-IV (1994), which have now become established entities. However, only a few clinical studies paying attention to the classification were carried out until recently. The brain morphological study as well as the gene analysis shows a similar tendency to the clinical investigation, and medical evidence for the pharmacotherapy of BP-II is still lacking. The recent data suggest that BP-II may be more prominent than BP-I in the subclinical depressive state, number of mood episodes, ratio of rapid-cyclers and co-morbidity of other psychiatric disorders, in spite of the low degree of the severity indicated by a lower incidence of psychotic symptoms, number of admissions and impairments of cognition. Bipolar disorders may consist of heterogeneous subtypes discriminated by various elements such as age at onset of the diseases. Without any conclusive result regarding the difference between the two types based on the age at onset, the ratio of the early onset may be higher in BP-II than in BP-I, which may influence the general features of each type.
 In the biological approach to bipolar disorders, the MRI technology demonstrated that there was a significant decrease in the brain volume in the subcortical regions, i. e., the hippocampus and amygdala, only in patients with BP-I, and that the prominently reduced volume in the subfields of the hippocampus was also observed only in the BP-I patients. In addition, a genome-wide analysis study found a significant change in the heritability, the genetic correlation and the polygenetic scoring of schizophrenia risk alleles between the types of bipolar disorders.
 On the other hand, the longitudinal clinical course of bipolar disorders can be hypothetically divided into several stages developing a neuroprogressive process similar to that of schizophrenia. It is an important issue to verify the validity of the hypothetical concept during the course of BP-II. Consequently, further studies are required to clarify the overall characteristics of the bipolar disorders by clinical and biological strategies keeping the diagnostic discrimination of the two types, and it may be indispensable to expand the comparison with other psychiatric disorders such as schizophrenia.
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