OBJECTIVES: To explore the possible comorbidities associated with Overactive Bladder (OAB), and to estimate the resource utilization pattern and cost of OAB in a Medicaid population. METHODS: A retrospective case-control matching study was performed. Five thousand five hundred seven continuously enrolled Medi-Cal patients who were diagnosed with OAB and received OAB prescriptions from 1999 to 2001 were 1:2 matched based on age, gender, race, and residence county. Annual hospital utilization and cost were calculated for both OAB and matched control cohorts, and prevalence ratios (PR, OAB over control) for 34 ICD-9 based comorbidity measures from AHRQ were examined. RESULTS: Out of 34 comorbidities, 13 occurred in OAB patients at least twice as much as in general Medicaid population: paralysis (PR=10.46), urinary track infection (UTI, PR=3.74), other neurological (PR=2.79), peripheral vascular disorder (PR=2.42), valvular disease (PR=2.32), arrhythmias (PR=2.31), atopic dermatitis (PR=2.19), blood loss anemia (PR=2.14), depression (PR=2.10), pulmonary circulation disorder (PR=2.10), hypothyroidism (PR=2.08), peptic ulcer disease including bleeding (PR=2.08), and deficiency anemias (PR=2.05). Among these diagnoses, obesity, UTI, dermatitis and depression are known comorbidities related to OAB. OAB patients had much higher annual resource utilization than the matched control group: physician office encounters (27.39 vs. 2.70, P<0.0001), and an emergency room visits (0.26 vs. 0.04, P<0.0001). OAB patients had approximately two-fold higher costs than the control cohort for pharmacy and medical services: $3319.85 vs. $1560.06 (P<0.0001), $4754.86 vs. $2592.68 (P<0.0001). CONCLUSIONS: OAB patients who received drug treatment incurred a heavy economic and resource burden to the California Medicaid program. Comorbid conditions were much more prevalent in the OAB cohort than in the general Medicaid population.