The twenty largest multi-county SMSAs in the U.S. were studied for trends in the suburbanization of medical practice from 1959 to 1974. Fifteen of them allowed a distinction between suburbs and central city on the county level according to criteria developed from a New York City pilot study. In all SMSAs, the density of hospital-based physicians was higher in central cities than in suburbs and still increasing more rapidly there, though hospital practice is also starting to grow in the suburbs at an appreciable rate. The same trends were found for physicians in non-patient care activities. Office based specialists in large Northeastern SMSAs increased much more rapidly in the urbanized suburbs than in the central cities. In the Southern and Western SMSAs, on the contrary, office-based specialists continued to increase rapidly in the central cities despite concentrations already far greater than in the Northeast. Suburban densities were growing more slowly in these areas. General practitioners declined uniformly everywhere, apparently simply by attrition. Implications of these trends for health care are discussed.