BACKGROUND: This is the first prospective longitudinal study, to our knowledge, of the natural history of the weekly symptomatic status of bipolar II disorder (BP-II). METHODS: Weekly affective symptom status ratings for 86 patients with BP-II were based on interviews conducted at 6- or 12-month intervals during a mean of 13.4 years of prospective follow-up. Percentage of weeks at each symptom severity level and the number of shifts in symptom status and polarity were examined. Predictors of chronicity for BP-II were evaluated using new chronicity measures. Chronicity was also analyzed in relation to the percentage of follow-up weeks with different types of somatic treatment. RESULTS: Patients with BP-II were symptomatic 53.9% of all follow-up weeks: depressive symptoms (50.3% of weeks) dominated the course over hypomanic (1.3% of weeks) and cycling/mixed (2.3% of weeks) symptoms. Subsyndromal, minor depressive, and hypomanic symptoms combined were 3 times more common than major depressive symptoms. Longer intake episodes, a family history of affective disorders, and poor previous social functioning predicted greater chronicity. Prescribed somatic treatment did not correlate significantly with symptom chronicity. Patients with BP-II of brief (2-6 days) vs longer (> or =7 days) hypomanias were not significantly different on any measure. CONCLUSIONS: The longitudinal symptomatic course of BP-II is chronic and is dominated by depressive rather than hypomanic or cycling/mixed symptoms. Symptom severity fluctuates frequently within the same patient over time, involving primarily symptoms of minor and subsyndromal severity. Longitudinally, BP-II is expressed as a dimensional illness involving the full severity range of depressive and hypomanic symptoms. Hypomania of long or short duration in BP-II seems to be part of the same disease process.