National attention has focused on the goal of attaining 50% primary care practitioners to facilitate patient access and cost-effectiveness. To determine how long it might take to achieve this goal, we used the Bureau of Health Professions' aggregate physician supply model to forecast the generalist-specialist balance. Assuming that 30% of graduates will enter generalist practice after 1993 (the percentage in the mid-1980s), the number of generalists would increase from 174,940 in 1990 to 232,000 in 2040 (77 per 100,000 population), but the percentage would remain at about 30%; specialists would continue to make up about 70% of all active physicians, but their total number would grow from 345,600 to 537,000 (178 per 100,000 population). If 50% of graduates were to enter generalist practices, by the year 2040 the number of generalists would grow to 373,000, or 124 per 100,000 (48.4% of all physicians). If entry into generalist practice falls to 20%, as suggested by recent medical student preferences, the number of generalists would peak at 192,000 (26.4%) in 2010 and would fall to 160,000 (21%) by 2040, resulting in 53 generalists and 201 specialists per 100,000 population. We discuss the implications of these findings on aggregate physician supply and on policy initiative affecting the ratio of generalists to specialists. Reform proposals affecting the specialty mix should clearly identify the desired future ratio of generalists and specialists per capita.