BACKGROUND: In recent years, studies of the risk of recurrence in affective disorder in relation to the number of prior episodes have given contradictory results. METHOD: Survival analysis was used to calculate the rate of recurrence after successive episodes in a case register study including all hospital admissions with primary affective disorder in Denmark during 1971-1993. A total of 20,350 first-admission patients were discharged with a diagnosis of affective disorder, depressive or manic/cyclic type. RESULTS: The rate of recurrence increased with the number of previous episodes in both unipolar and bipolar disorder. Initially, the two types of disorders followed markedly different courses, but later in the course of the illness the rate of recurrence was the same for the two disorders. CONCLUSIONS: The course of severe unipolar and bipolar disorder seems to be progressive in nature despite the effect of treatment.