Objective. To examine the extent to which health plan quality measures capture physician practice patterns rather than plan characteristics. Data Source. We gathered and merged secondary data from the following four sources: a private firm that collected information on individual physicians and their health plan affiliations, The National Committee for Quality Assurance, InterStudy, and the Dartmouth Atlas. Study Design. We constructed two measures of physician network overlap for all health plans in our sample and linked them to selected measures of plan performance. Two linear regression models were estimated to assess the relationship between the measures of physician network overlap and the plan performance measures. Principal Findings. The results indicate that in the presence of a higher degree of provider network overlap, plan performance measures tend to converge to a lower level of quality. Conclusions. Standard health plan performance measures reflect physician practice patterns rather than plans' effort to improve quality. This implies that more provider-oriented measurement, such as would be possible with accountable care organizations or medical homes, may facilitate patient decision making and provide further incentives to improve performance.