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Introduction and Objective: Age at onset in bipolar disorder is related to prognosis and to treatment response. However, it is not clear if there are three or two subgroups in relation to age at onset. The objective of this study is to analyze the number of subgroups in relation to age at the beginning of the disease in a representative sample of bipolar I patients and to compare the subgroups in relation to clinical variables., Method: We included 169 patients diagnosed with bipolar I disorder. Normal mixture analysis was performed. The subgroups of patients formed above were compared regarding clinical characteristics. Patients were followed-up during six years., Results: We found three ages at onset subgroups. The early onset group (18.2±2 years) included 34% of the patients. The second group (26.1±5.5 years) included 44% of the patients. The third group (50.9±9.1 years) included 22% of the patients. Early and intermediate onset groups were not significantly different, and had more family history of affective disorders, more psychotic symptoms, more history of suicide attempts and more history of drug abuse history than the late onset group., Conclusions: Our results suggest that there are three groups of age at onset but early and intermediate groups are similar in relation to clinical variables. The late onset group includes almost a quartile of patients and has different clinical profile., (Copyright © 2009 Sociedad Española de Psiquiatría and Sociedad Española de Psiquiatría Biológica. Published by Elsevier Espana. All rights reserved.)