OBJECTIVES: To determine the prevalence of patients with bipolar disorder (BD) and compare their annual health care costs to patients with other mental health disorders, in a large United States managed care organization. METHODS: This was a retrospective claims analysis of approximately 1.4 million commercial health plan members with mental health benefits. Adults with a primary or secondary diagnosis of a mental health disorder in 2002 were identified and the prevalence of BD calculated. Those continuously enrolled throughout 2003 (follow-up period) were stratified to one of two cohorts: “BD” (Bipolar Disorder) or “OMHD” (Other Mental Health Disorder). Patient demographics, pharmacological treatments, and health care charges were compared between cohorts, adjusting for potential confounding factors of age, gender, and comorbidity. RESULTS: During 2002, there were 6581 patients (mean age 40.3 years; 65.7% female) with BD, yielding an overall prevalence rate of 4.68 per 1000 members. Among the 64,434 continuously enrolled mental disorder patients in 2003, 3,043 (4.7%) were classified as “BD” and 61,391 (95.3%) were classified as “OMHD”. Patients in the “BD” group were younger (41.7 vs. 43.0 years; p<0.0001) with higher Charlson comorbidity index (0.56 vs. 0.47; p<0.0001) compared to the “OMHD” group. Less than half (38.3%) of “BD” patients received a mood stabilizer (lithium, valproate, or carbamazepine) and 20.0% received no psychotropic medication. Adjusted pharmacy, medical, and total health care charges were higher in the “BD” group compared to “OMHD”: $2,641 vs. $1,071, $13,419 vs. $8,422, and $16,059 vs. $9,493 respectively (p<0.0001 for all three comparisons). CONCLUSIONS: Compared to the national prevalence rate of 1%, BD may be under-diagnosed, and pharmacologic therapy underutilized within managed care. Patients with BD incurred greater health care charges compared to other mental health disorders. Efforts aimed at improving diagnosis and treatment of BD may optimize care and cost of managing this patient population.