The age of onset in bipolar affective disorder was determined in 200 hospitalized patients. The mean age of their first affective syndrome was 28.3 years, and the mean age of first hospitalization was 30.8 years. However, the median age for first affective syndrome was 23 years (26 years for first hospitalization), and the most common age of onset was 15-19 years. Those patients with an early onset, especially if they were first hospitalized for mania, were most likely to have received a diagnosis of schizophrenia.