Background: Copayments (copays) for prescription drugs are a common policy among state Medicaid programs. Research exploring the effects of copays on pharmacy and health care utilization in Medicaid patients is limited, especially among patients with chronic disease. Objectives: The goal of this research was to quantify the impact of a copay policy for prescription drugs on medication and health services utilization overall and among subjects with several common chronic diseases enrolled in a state Medicaid program. Research Design: Using aggregated pharmacy claims, segmented linear regression models were used to evaluate changes in overall and disease-specific pharmacy utilization after implementation of a copay policy. Trends in emergency department encounters, office visits, and hospitalizations were used to evaluate the impact of this policy on unintended consequences. Utilization among cohorts of patients with several chronic conditions were analyzed to determine if a differential response existed by drug indication. Results: After copay implementation, utilization of prescription drugs declined significantly by 17.2% (P < 0.0001). This pattern was observed at varying degrees for all drug classes investigated. Rates of emergency department encounters, office visits, or hospitalizations did not increase after the policy was introduced. Subjects with diabetes, respiratory disease, and schizophrenia immediately reduced their use of nonindicated drugs significantly more than drugs indicated for their condition. Conclusions: Among Medicaid recipients, nominal copays are associated with significant reductions in use of clinically important drug classes. However, patients with chronic disease exhibited a differential response depending on the disease indication of the drug class.