OBJECTIVE: To examine mental health medical services utilization and costs for patients in the depressive phase of bipolar. METHODS: This retrospective study included a cohort of 1419 patients who had 3 consecutive years of data (majority between 1999 and 2001), received a diagnosis of bipolar depression (ICD codes), and utilized one of three types of medical services; room and board, medical/surgical, and ancillary services associated with a mental disorder diagnosis. Medical service utilization and costs were observed for a one-year period post diagnosis index. RESULTS: Twenty-one percent of patients incurred hospitalization charges, averaged two admissions per year and accounted for 50% of medical service costs. Average annual medical service costs for patients who continue their initial treatment are $1950. These cost more than triple ($6570) for patients who incur three or more switches in a year. Additionally, the largest increase in medical service costs is from the initial treatment ($1950) to the first switch ($3364). Bipolar depressed patients who received no psychotropic medication incurred $3903 of medical service costs. CONCLUSION: Patients with more "stabilized" treatment had lower medical service costs than those patients who experienced switches in pharmacologic treatment. Additionally, non-medicated bipolar depressed patients incur high annual costs to the managed care organization.