Data on 108 hospitalized bipolar I women were analyzed to characterize those whose course was marked with at least one mixed episode (i.e. an episode with concomitant manic and depressed features) on the basis of various anamnestic and cross-sectional clinical features in comparison with those without mixed episodes. Our data revealed a later age of appearance of the first mixed episode in the course of bipolar illness with a tendency to recur true to type; greater prevalence of mood incongruent psychotic features; lower frequency of hyperthymic temperament; and familial depressive, rather than bipolar, disorders. These characteristics tend to identify the mixed state as a distinct longitudinal pattern of manic-depressive illness.