Several studies have concluded that specialists form a hidden system for primary care delivery. These studies, however, assume that a specialist who provides the majority of care is the primary care physician. Using data for a one-year period from 2,752 people enrolled in the RAND Health Insurance Experiment, the authors examine the validity of this conclusion. They compare how three different definitions of a primary care physician affect estimates of who provides primary care: (1) the physician who delivered the "majority of care" (34 percent were specialists); (2) the physician who was designated to receive results of a multiphasic screening examination (12 percent were specialists); and (3) the physician who treated common problems (9 percent were specialists). Thus, the contribution made by specialists is overestimated threefold when the "majority of care" concept is used to define primary care. Definitions of a primary care physician must be more specific, must include tasks frequently associated with primary care, and must take account of patients' perceptions of who functions as their primary care physician.