Objective: To assess impact of change in prescription copay on change in medication utilization among patients taking three common brand name medications: atorvastatin for dyslipidemia, pioglitazone for diabetes, or valsartan for hypertension. Methods: The Medstat MarketScan database was used to identify patients taking a study drug (atorvastatin, pioglitazone, or valsartan) in both 2003 and 2004. Patients had to have continuous pharmacy coverage, at least one fill in each year, and be <65 years of age. The total copay for study drug was divided by the total number of prescription fills to calculate average copay for each patient in each year. Regression and partial correlation analysis was used to estimate the association between changes in copay and days supply, adjusting for age and gender. Results: Among 9342 valsartan patients, 4622 (49%) patients had an increase in average copay and for these patients there was on average 32 fewer days on drug in 2004 compared to valsartan patients without an increase in copay (Spearman Rho= -0.14, p-value<0.01). There was also a negative association between copay and days supply in patients receiving atorvastatin or pioglitazone: 15 days less on pioglitazone and 18 days less on atorvastatin for patients having an increase in average copay for 2004. Among patients using mail order prescriptions with fills for >30 day supply the impact of copay on days supply was less but still statistically significant. Conclusion: Policy and benefit decision makers need to consider the impact of patient copay on persistence for chronic diseases such as diabetes, hyperlipidemia, and hypertension.