Objective: Childhood obesity as a known risk factor associated with bipolar disorder complicates its treatment. The purpose of this study is to assess treatment costs and comorbidities associated with obesity in children and adolescents with bipolar disorder. Methods: Based on a multi-state managed care medical claims database (PharMetrics), a total of 9895 children and adolescents (6Results: A total of 341 (3.4%) children or adolescents received an obesity diagnosis during the study period. The average annual treatment costs were $7,481 (SD±8,371) for patients experiencing obesity and $5,364 (SD±15,322) otherwise. A total of 4204 (42.5%) patients received atypical antipsychotics, 2237 (22.6%) with lithium, and 5,890 (59.5%) with other anticonvulsants. Being obesity is associated with atypical antipsychotic use (odds ratio[OR]=1.49, 95% confidence interval [CI] 1.18-1.88), and key comorbidities like diabetes mellitus (OR=3.40, 95%CI 1.96-5.89) and hypertension (OR=4.41, 95%CI 2.70-7.20). Higher treatment cost is associated with the use of atypical antipsychotics (p<0.0001), hospitalization (p<0.0001), ER visit (p<0.0001), and some key comorbidities like diabetes mellitus (p<0.0001) and substance abuse disorder (p<0.0001). Conclusion: Higher treatment costs are associated with obesity in children or adolescents with bipolar disorder. Metabolic complications should be considered by clinical practitioners when prescribing medication in this population.