In this report the data from medical history questionnaires, screening examinations, insurance claims, and a face-to-face physician interview were used to examine the quality of ambulatory care received for 17 chronic conditions by a general population of 5986 adults (less than or equal to 65) and children (less than or equal to 14) enrolled in the RAND Health Insurance Experiment. Subjects in six U.S. sites were randomly assigned to insurance plans that were free or that required cost sharing, or in one site to an HMO. Quality-of-care criteria--both process (what was done to patients) and outcome (what happened to them)--were developed. Overall, 81% of outcome criteria and 62% of process criteria were met. Physicians interviewed patients with selected conditions at the Experiment's end to evaluate care. They suggested that approximately 70% of patients should have their current therapy changed, but only 30% of patients would obtain more than minor improvement from such a change. Clinically meaningful plan differences in quality of care were observed only for the process criteria dealing with the need for a visit (free plan compliance 59%; cost sharing compliance 52%). Quality of care for the poor was slightly worse than for the nonpoor and persons randomized to an HMO had slightly better overall quality of care than those in the fee-for-service system. Substantial improvements in the quality of the process of care could be made, but impact on outcome may be small. Results of the analysis suggest the need for development of clinical models to test the relationship between specific process criteria and improvements in outcome.