A remarkable development in primary care is the recent emergence of a new class of health professional: nurse practitioners and physician's assistants. These practitioners diagnose and treat a wide variety of medical problems, usually with supervision by physicians. Their clinical competence has been evaluated in over 40 studies. Twenty-one studies in which care given by nurse practitioners or physician's assistants was directly compared with that given by physicians are analyzed. These studies show that nurse practitioners and physician's assistants provide office-based care that is indistinguishable from physician care. Because these studies were limited in scope, there is no experimental basis for extending this conclusion to care given outside the office, care that is unsupervised, or care of the seriously ill patient.