Pay-for-performance (P4P) has become one of the dominant approaches to improving quality of care, yet few studies have evaluated its effectiveness. We evaluated the impact on quality of all P4P programs introduced into physician group contracts during 2001-2003 by the five major commercial health plans operating in Massachusetts. Overall, P4P contracts were not associated with greater improvement in quality compared to a rising secular trend. Future research is required to determine whether changes to the magnitude, structure, or alignment of P4P incentives can lead to improved quality.