This study examines rates of co-morbid mental disorders and indicators of the course of illness among individuals with bipolar disorder and cannabis use disorders (CUD). Data were drawn from the National Epidemiological Survey of Alcohol and Related Conditions (NESARC Wave 1. 2001-2002), a nationally representative sample of adults living in the United States. Among individuals with lifetime prevalence of bipolar disorder (N=1905) rates of CUD in the past 12 months were 7.2%, compared to 1.2% in the general population. Logistic regression models adjusting for sociodemographic variables indicated that individuals with bipolar disorder and co-occurring CUD were at increased risk for nicotine dependence (Adjusted Odds Ratio (AOR)=3.8), alcohol (AOR=6.6) and drug (AOR=11.9) use disorders, as well as antisocial personality disorder (AOR=2.8) compared to those without CUD. Among individuals with co-occurring CUD, age of onset of bipolar disorder was significantly lower and median number of manic, hypomanic and depressive episodes per year was significantly greater compared to individuals without CUD. Co-occurring CUD is associated with significant co-morbidities and a more severe course of illness among individuals with bipolar disorder. Comprehensive evaluation of patients with bipolar disorder should include a systematic assessment of CUD. (C) 2012 Elsevier Ireland Ltd. All rights reserved.