A review of two centuries' literature on the natural history of bipolar disorder, including modern naturalistic studies and new data from a lifelong follow-up study of 220 bipolar patients, reaches the following conclusions: the findings of modern follow-up studies are closely compatible with those of studies conducted before the introduction of modern antidepressant and mood-stabilizing treatments. Bipolar disorder has always been highly recurrent and considered to have a poor prognosis. Bipolar patients who have been hospitalized spend about 20% of their lifetime from the onset of their disorder in episodes. Fifty percent of bipolar episodes last between 2 and 7 months (median 3 months). The intervals between the first few episodes tend to shorten; later the episodes return at an irregular rhythm of about 0.4 episodes per year with high interindividual variability. Switches from mania into mild depression and from depression into hypomania were frequently reported in the 19th century and the first half of the 20th. Antidepressant and antimanic drugs have to be given as long as the natural episode lasts. Given the poor outcome of bipolar disorders found in naturalistic follow-up studies and our lifelong investigation, intensive antidepressant, antimanic, and mood-stabilizing treatments are required in most cases. Despite modern treatments the outcome into old age is still poor, full recovery without further episodes rare, recurrence of episodes with incomplete remission the rule, and the development of chronicity and suicide still frequent.