Ambulatory care, accounting for over half a billion visits to physicians per year, is a major component of the health-care system and is the core of primary health care. This study uses data from the National Ambulatory Medical Care Survey to describe the most common problems seen in an ambulatory-care setting, to identify the medical specialties that provide the greater part of this care, and to characterize the major specialties in terms of the diagnoses in the patients who constitute their ambulatory practice. Fifteen diagnosis clusters account for 50 per cent of all ambulatory-care visits; only 8 of the 28 specialties account for a substantial amount (more than 25 per cent) of the ambulatory care rendered to patients with any of these 15 diagnoses. General and family physicians, general internists, and general pediatricians account for 65.9 per cent of all outpatient visits to physicians for the 15 most common problems; general and family physicians alone are responsible for more than half this total. The individual specialties differ markedly in the diagnostic and demographic variety of their outpatient workload. These differences have important implications for the training of physicians and the organization of their practices.