Nine Professional Standards Review Organizations jointly conducted an areawide medical care evaluation study of Cesarean births. Of 5,002 Cesarean births studied in 158 hospitals, only 1.1 per cent were judged to be unjustified procedures by local physician peer review. Although there was substantial geographic variation in rates of Cesarean birth (11.0%-18.3 per cent), no statistically significant pattern of variation was identified. Maternal outcomes and infant outcomes were generally good. This study demonstrates the potential for use of PSRO data in studying major issues in health care, as well as the practical constraints of such studies.