Do bipolar I and bipolar II subtypes represent two different forms of affective illness in terms of course? To test this working hypothesis, a comparison was made of data on course of bipolar I (n = 84) and bipolar II (n = 16) subgroups from 100 consecutive in-patients diagnosed as bipolar affective disorder. The two subgroups differed significantly regarding course. The patients of the bipolar II group had significantly later first admissions, more episodes, more prolonged mean duration of phase and shorter mean duration of intervals. In bipolar I group, a greater frequency of manic phases was found.