Little is known about the prevalence of focused expertise (special areas of expertise within a clinical field) among physicians, yet such expertise may influence how care is delivered. We surveyed general internists, pediatricians, cardiologists, infectious disease specialists, and orthopedic surgeons to describe the prevalence of focused expertise and identify associated physician and practice characteristics. About one quarter of generalists and three quarters of specialists reported a focused expertise within their primary specialty. Hospital-based physicians more often reported such expertise, and physicians reimbursed by capitation less often reported expertise. Learning how focused expertise affects processes and outcomes of care will contribute to decisions about physician training and staffing of medical groups. T he rapid expansion of medical knowledge and technology has fueled the growth of medical specialization over recent decades. 1 Many physicians have specialized their practice beyond traditional disciplines. Cardiologists, for example, may concentrate in areas such as echocardiography, electrophysiology, or interventional cardiology. The expansion of specialty boards, including increasing opportunities for certification within the traditional specialties, has further encouraged this trend toward more specialized training. 2 Expertise within generalist fields has been described less well than within specialist disciplines. In 1994, the American College of Physicians Task Force on Physician Supply envisioned general internists of the future as both experts in the general care of adults and local authorities with more specialized expertise, either in traditional, organsystem-based subspecialties or in new areas of internal medicine such as women's health and geriatrics. 3 One academic medical center now offers a fourth year of internal medicine residency training to foster focused expertise in the practice of general internal medicine. 4 Similar trends are evident in pediatrics, with the emergence of fellowship training and certification in fields such as adolescent medicine and behavioral-developmental pediatrics. 5,6 Generalists with focused expertise may be particularly useful in managed care organizations or in geographic areas with fewer specialists. A group of primary care physicians, each with complementary expertise, might share knowledge and skills and thus require fewer consultations with specialists.In this study, we sought to understand the prevalence and distribution of self-reported expertise in generalist and specialist fields so that future studies can investigate its effect on processes and outcomes of care. We asked physicians in 5 specialties whether they have focused expertise within their primary specialty. We also examined physician and practice characteristics associated with reporting focused expertise.