Although numerous changes are apparent in the US health care system, little is known about how these changes have altered the work of primary care physicians. We analyzed a nationally representative sample of 136,233 adult office visits to general internists, general practitioners, and family physicians contained in the 1978 through 1981, 1985, and 1989 through 1994 National Ambulatory Medical Care Surveys. Annual sample sizes varied between 5662 and 19,977 visits. Measures included the characteristics of patients presenting to primary care physicians, physician activities during these visits, and the disposition of the visits to primary care physicians. Visits to primary care physicians have diminished as a proportion of all adult visits from 52% in 1978 to 41% in 1994. Dramatic trends in adult primary care included the growing racial or ethnic diversity of patients, the doubling (since 1985) of health maintenance organization coverage, increased provision of prevention services, changes in the most common medications, and an 18% increase in the duration of adult visits to primary care physicians. Trends in primary care practice reflect changes in society and in the US health care system, including demographic changes, an emphasis on prevention, and the growth of managed care. The increasing role of managed care, with its emphasis on increased productivity, appears at odds with primary care physicians' increasing responsibility for prevention and the associated increase in the duration of primary care visits.