In 1993, there were 4,355 active, postresident physicians in the United States who, according to the American Medical Association (AMA) Physician Masterfile, had their primary specialty in either adult nephrology or pediatric nephrology. These renal physicians constituted 0.8% of the active postresident physician population, and there were 1.67 renal physicians for every 100,000 people in the United States. The population of renal physicians has grown at a significantly greater rate than the physician population as a whole. The number of renal physicians increased more than 10-fold between 1970 and 1993, and it increased by 19% between 1990 and 1993. In recent years, the growth of the renal physician population has been bolstered by the large number of new international medical graduates (IMGs) entering the United States, who have entered nephrology at a much higher rate than entering US medical graduates (USMGs). The future growth of the renal physician population will be determined by a wide variety of factors, including future trends in specialty selection and whether policies are implemented to reshape the physician workforce. The projection analysis of the future supply of adult nephrologists considers three different scenarios. First, if the production of new nephrologists remains at status quo, the supply of adult nephrologists will increase 101% between 1993 and 2010. However, it is possible that the number of entrants into the adult nephrologist population will increase because of the large influx of IMGs in US residency programs in recent years. If new IMGs continue to enter adult nephrology at their current rates and if this heightened level of entry is sustained, the supply of adult nephrologists will increase 134% between 1993 and 2010. Finally, if proposals are implemented to reduce the number of first-year residency positions to 110% of the number of new graduates of US medical schools and to raise the proportion of new physicians entering primary care to 50%, the supply of adult nephrologists will increase 62% between 1993 and 2010. The demand for nephrologists' services is likely to grow in the near future. The prevalence rate of end-stage renal disease (ESRD) more than doubled between 1984 and 1993, and a continuation of this trend will expand the need for more nephrologists. The long-term growth of the ESRD population will also be sustained, to some extent, by the aging of the US population, because the prevalence of ESRD is relatively high in the older age categories. However, long-term increases in the demand for nephrologists may be restricted by the growth of managed care, the use of nonphysician providers, and the implementation of other cost-cutting measures that impact the delivery of services to the ESRD population.