DSM-III-R criteria, applied retrospectively in a research-oriented psychiatric clinic, identified patients (N = 146) with a mood disorder and a seasonal pattern of recurrence (seasonal mood disorder). The seasonal mood disorder syndrome was not rare (10% of all mood disorders); diagnostic distribution was as follows: recurrent depression, 51%, and bipolar disorder, 49%, with 30% of the latter having mania (bipolar disorder type I) and 19% having hypomania (bipolar disorder type II). Most patients were women (71%); onset age averaged 29 years, with a mean of eight cycles in 12 years of illness; mean episode duration was 5.0 months. Mood disorder was found in a high proportion (68%) of the families. All but one patient followed one of two seasonal patterns in equal frequency: type A, fall-winter depression with or without spring-summer mania or hypomania; and type B, spring-summer depression with or without fall-winter mania or hypomania. Both types showed consistent times of onset and remission. These results emphasize that DSM-III-R seasonal mood disorder includes severe cases of recurrent depression and bipolar disorder and support a distinction between two seasonal subtypes.