OBJECTIVE: This study clarified the early characteristics of substance use disorders in patients with first-episode bipolar I disorder. METHOD: The authors evaluated substance use disorders, associated factors, and clinical course, prospectively, in the first 2 years of DSM-IV bipolar I disorder with standardized methods. RESULTS: Baseline substance use disorder was found in 33% (37 of 112) of the patients at baseline and in 39% at 24 months. Anxiety disorders were more frequent in the patients with than without substance use disorder (30% and 13%, respectively). Associations of alcohol dependence with depressive symptoms and cannabis dependence with manic symptoms were suggested. Patients using two or more substances had worse outcomes. CONCLUSIONS: Since substance use disorders were frequent from the beginning of bipolar I disorder and were associated with anxiety disorders and poor outcome, early interventions for substance use disorder and anxiety might improve later outcome.