OBJECTIVES: To estimate the rates and costs of all-cause and psychiatric-related hospitalizations and ER visits among patients with bipolar I disorder.  METHODS: Retrospective cohort analysis of multistate Medicaid patients who were aged ≥18 years, had ≥1 inpatient or ≥2 outpatient medical claims indicating bipolar I disorder (ICD-9-CM codes 296.0X-296.1X, 296.4X-296.7X), and filled ≥1 prescription for an antipsychotic medication between January 1, 2004, and December 31, 2006. Patients were followed for 1 year from the date of first, or index, antipsychotic prescription. Patients were required to be continuously eligible for Medicaid without dual Medicare eligibility from 1 year before (baseline) through 1 year after (follow-up) index, and were required to receive ≥1 additional antipsychotic claim during follow-up to ensure a treated population. Descriptive statistics were generated on the use and costs (proxied by Medicaid payments) of hospitalizations and ER visits, both all-cause and psychiatric-related (ICD-9-CM 290.XX-319.XX). RESULTS: 9410 patients met study eligibility criteria. Mean (±SD) patient age was 38.0 (±11.9) years, 74% were female, and 75% were white. Approximately 31% and 57% had preexisting diagnoses of substance abuse and psychiatric conditions, respectively. During follow-up, 40% of patients were hospitalized for any reason (37% with a psychiatric diagnosis). Patients had an average (±SD) of 0.9 (±1.7) all-cause and 0.8 (±1.6) psychiatric-related hospitalizations. All-cause and psychiatric-related ER visits occurred in 67% and 29% of patients, with an average (±SD) of 6.1 (±10.9) and 0.9 (±2.7) visits per patient, respectively. Average (±SD) costs were $6986 (±$10,374) and $6916 (±$10,288) for all-cause and psychiatric-related hospitalizations, and $158 (±$218) and $256 (±$416) for all-cause and psychiatric-related ER visits, respectively. CONCLUSIONS: Hospitalizations, ER visits, and costs were substantial among patients with bipolar I disorder. Studies investigating predictors of hospitalizations and ER visits in this patient population are warranted. Supported by funding from Ortho-McNeil Janssen Scientific Affairs, LLC