We implemented the most frequently used form of quality assurance activity: abstracting information on the quality of patient care from medical records and communicating findings to providers in 16 ambulatory care groups. Site providers accepted the evaluation criteria, agreed that deficiencies in care were detected, and, for some medical tasks, effected improvements in care. Direct costs in 1980 dollars for the quality assurance cycle including data system development were $46 per evaluated case. Per-case costs varied considerably among tasks, decreased with larger numbers of cases and as experience grew, and were reduced through computerization. Measured costs were high due to: a demanding research design; our extended accounting of direct, indirect, and induced costs; and the substantial resource requirements of rigorously performed evaluations.