Background: To our knowledge, this is the first prospective natural history study of weekly symptomatic status of patients with bipolar I disorder (BP-I) during long-term follow-up. Methods: Analyses are based on ongoing prospective follow-up of 146 patients with Research Diagnostic Criteria BP-I, who entered the National Institute of Mental Health (Bethesda, MD) Collaborative Depression Study from 1978 through 1981. Weekly affective symptom status ratings were analyzed by polarity and severity, ranging from asymptomatic, to subthreshold levels, to full-blown major depression and mania. Percentages of follow-up weeks at each level as well as number of shifts in symptom status and polarity during the entire follow-up period were examined. Finally, 2 new measures of chronicity were evaluated in relation to previously identified predictors of chronicity for BP-I. Results: Patients with BP-I were symptomatically ill 47.3% of weeks throughout a mean of 12.8 years of follow-up. Depressive symptoms (31.9% of total follow-up weeks) predominated over manic/hypomanic symptoms (8.9% of weeks) or cycling/mixed symptoms (5.9% of weeks). Subsyndromal, minor depressive, and hypomanic symptoms combined were nearly 3 times more frequent than syndromal-level major depressive and manic symptoms (29.9% vs 11.2% of weeks, respectively). Patients with BP-I changed symptom status an average of 6 times per year and polarity more than 3 times per year. Longer intake episodes and those with depression-only or cycling polarity predicted greater chronicity during long-term follow-up, as did comorbid drug-use disorder. Conclusions: The longitudinal weekly symptomatic course of BP-I is chronic. Overall, the symptomatic structure is primarily depressive rather than manic, and subsyndromal and minor affective symptoms predominate. Symptom severity levels fluctuate, often within the same patient over time. Bipolar I disorder is expressed as a dimensional illness featuring the full range (spectrum) of affective symptom severity and polarity.