OBJECTIVES: To examine psychiatric-related health care costs and inpatient utilization among patients with type I bipolar disorder who relapse frequently within a large Medicaid database.  METHODS: A large multistate Medicaid claims database (MarketScan®) was used to identify patients aged 18 to 64 years with type I bipolar disorder; ≥1 prescription claim for lithium, an anticonvulsant, or an antipsychotic between January 1, 2004 and December 31, 2005; and ≥24 months’ continuous enrollment. Frequent relapse (FR) was defined as ≥2 clinically significant events (CSEs) occurring during a 12-month identification period beginning with the initial CSE (if any). CSEs included emergency room (ER) visits or inpatient hospitalizations (IH) with a principal diagnosis of bipolar disorder or a change in bipolar disorder medication(s). Patients were followed for a subsequent 12-month period to evaluate health care utilization and associated costs. Generalized linear modeling was used to estimate the impact of FR on psychiatric-related health care costs; logistic regression was used to estimate the impact of FR on psychiatric IH and ER visits. RESULTS:  Of 5527 patients with type I bipolar disorder, 53% relapsed frequently. Of the patients with FR, 75% were female. Close to one-third (29%) of patients who relapsed frequently in the identification period also relapsed frequently in the follow-up period. During the 12-month follow-up period, patients with FR had higher adjusted per-patient psychiatric-related health care costs (mean $6014 vs $3495; P<0.001), 3.7 times greater odds of psychiatric IH (P<0.01), and 3.1 times greater odds of psychiatric-related ER visits (P<0.01) than patients who did not relapse frequently. CONCLUSIONS: Medicaid patients with type I bipolar disorder who relapsed frequently had significantly higher adjusted psychiatric-related health care costs and greater odds of IH and ER events in a subsequent period than patients who did not relapse frequently. Supported by funding from Ortho-McNeil Janssen Scientific Affairs, LLC