OBJECTIVE: To assess the impact of increased prescription drug copayments on the therapeutic classes of drugs received and health status of the elderly. HYPOTHESES TESTED: Increased prescription drug copayments will reduce the relative exposure to, annual days use of, and prescription drug costs for drugs used in self-limiting conditions, but will not affect drugs used in progressive chronic conditions and will not reduce health status. STUDY DESIGN: Each year over a three-year period, one or the other of two well-insured Medicare risk groups in an HMO setting had their copayments per dispensing increased. Sample sizes ranged from 6,704 to 7,962. DATA SOURCES/DATA COLLECTION: Automated administrative data systems of the HMO were used to determine HMO eligibility, prescription drug utilization, and health status. ANALYSIS DESIGN: Analysis of variance or covariance was employed to measure change in dependent variables. FINDINGS: Relative exposure, annual days of use, and prescription drug costs for drugs used in self-limiting conditions and in progressive chronic conditions were not affected in a consistent manner across years by increases in prescription drug copayment. Health status may have been adversely affected. Larger increases in copayments appeared to generate more changes. CONCLUSIONS: Small changes in copayments did not appear to substantially affect outcomes. Large changes in copayments need further examination.