Objective: Obesity, a known risk factor associated with bipolar disorder, complicates its treatment, potentially adding significantly to overall treatment cost. The difference in bipolar treatment cost between obese patients and normal weight patients has not been adequately evaluated. The purpose of this study is to assess treatment costs and co-morbidities associated with obesity in Medicaid patients with bipolar disorder. Methods: From a multi-state managed care Medicaid claims database, a total of 13,242 patients with bipolar disorder during the period January 1, 1998 to December 31, 2002 were selected for this study. Patients had to be older than six years or younger than 65 at the first date of bipolar diagnosis. Annual treatment cost per patient was constructed as the sum of reimbursed amounts (in 2002 constant dollars) for hospitalizations, physician encounters, drugs, and other medical services. A log-linear regression analysis was used to assess factors influencing annual treatment costs. Logistic regression analysis was conducted to assess the association between overweight and related clinical factors. Results: A total of 3192 (24.1%) were treated with atypical antipsychotics, 1700 (12.8%) with lithium, and 4575 (34.5%) with other anticonvulsants. A total of 1064 (8.0%) patients received an obesity or overweight diagnosis during the study period. The average annual treatment costs were $11,780 (SD±15,290) for patients experiencing obesity and $8,546 (SD±17,971) otherwise. Other major determinants of higher treatment cost included bipolar I or NOS (p<0.0001), being female (p<0.0001), the use of anticonvulsants (p<0.0001) and atypical antipsychotics (p<0.0001), alcohol and substance abuse disorder (p<0.0001), and diabetes mellitus (p<0.0001). The risk of being overweight was statistically associated with co-morbidities like anxiety disorder (odds ratio [OR]=1.43, 95% CI: 1.25-1.64), hypertension (OR=3.02, 95% CI: 2.54-3.59), etc. Conclusion: Significant treatment costs are associated with obesity in patients with bipolar disorder. Metabolic complications should be considered by clinical practitioners when prescribing medication in this population.