The initial conception of manic depressive illness by Kraepelin included short and mild depressive and hypomanic states in the nosologic category of affective illness. The concept of recurrent brief depression was further confirmed in recent community and general practice studies. The diagnostic criteria for recurrent brief depression requires the presence of at least five of nine depressive symptoms analogous to the symptoms of major depression, yet a duration of less than two weeks (in general 1 to 3 days), a recurrence of at least 12 times a year, and the evidence of work impairment. The 1-year prevalence in the general population is about 5% and the lifetime prevalence 16%. Recurrent brief depression may develop into major depression and vice versa in about the same percentage of cases. Patients with combined major and recurrent brief depression are more severely affected, have a higher suicide attempt rate, and have an increased frequency of treatment--seeking than patients with only one condition. Recurrent brief depression is typically characterized by monthly occurring short-lived depressive episodes, with an erratic, relapsing and chronic course and significant suicide risk. The duration of these episodes in clinical samples was of 3-5 days on average, with a medium number of episode a year of 20. The onset of episodes is sudden and patients progress rapidly to the severe levels of depression. Resolution of the symptoms also tends to be rapid. The suddeness of the episodes may contribute to the relationship reported with suicidal behaviour. This is a disabling illness because of the frequency with which the episodes recur reported as erratic "irregularly regular".(ABSTRACT TRUNCATED AT 250 WORDS)