OBJECTIVE: Numerous studies have found that substantial increases in drug co-payments result in reductions in utilization and spending. A recent study found that significantly more members faced with substantially increased costs stopped statin therapy than a control group. High co-payments can influence the decision to start needed drug therapy as well as to discontinue it. This study analyzes population-based statin use rates as one way to look at the impact of high cost sharing on the decision to start therapy as well as to continue statin therapy. METHODS: The study included 15,937 statin users in 16 employer groups with average member cost share greater than 30% (HIGH) and 86,605 statin users in 48 employer groups with member share less than 15%. Employer groups with DAW penalties were excluded. LOW and HIGH share groups both had retail and mail coverage. Drug utilization and group eligibility data for 2003 came from Caremark Rx claims data system. HIGH and LOW groups were compared on age and gender. Age-specific population use rates were computed for 12 five-year age bands between 30 and 89. RESULTS: Age-specific population-based use rates for the HIGH share group averaged only 79% of the level of the LOW share group. For example, the statin use rate for 60-64 years of age was 279.1 per 1000 for the HIGH share group and 359.8 per 1000 for the LOW share group. CONCLUSIONS: High levels of member cost sharing impact population-based statin use rates. Additional educational efforts may be necessary to encourage use of preventive health care as members assume greater responsibility for the costs of medication.