Medicare's Physician Quality Reporting System (PQRS) is the largest quality-reporting system in the U.S. health care system and a basis for the new value-based modifier system for physician payment. The PQRS allows health care providers to report measures of quality of care that include both the process of care and physiological outcomes. Using a multivariate difference-in-differences model, we examine the relationship of PQRS participation to three claims-computable measures of inappropriate utilization of health care services and risk-adjusted per capita Medicare expenditures. The data are a national random sample of PQRS-participating providers matched to nonparticipating providers by zip code and caseload. We found few significant relationships in the overall analysis. However, the magnitude and statistical significance of the desirable associations increased in subgroups of providers and beneficiaries more prone to overutilization (e.g., males, older beneficiaries, beneficiaries treated in larger medical practices or by nonphysicians, and practices in rural areas), and among beneficiaries with heart conditions, diabetes, and eye problems.