What is the relationship between the market power of health plans, relative to providers, and the quality, intensity, and accessibility of health care? Using the household and physician surveys from the Community Tracking Study, the researchers studied the following questions: 1) Across markets, are concentrations of market share among a few health plans correlated with the clustering of providers into a few dominant organizations? 2) Do physicians in markets with a few dominant provider organizations report lower work hours and changes in compensation? 3) What are the relative effects of provider and health plan concentration on the intensity and specialty-orientation of health care? 4) What are the relative effects of provider and health plan concentration on the perceived quality and convenience of care? 5) How closely correlated across markets are the perspectives of physicians and patients? The objective of the study was to inform the policy and regulatory debate about the advantages and disadvantages of concentrated market power.