BACKGROUND International medical graduates (IMGs) have been a valuable resource for the United States physician workforce, and their contribution to the United States workforce is likely to increase. OBJECTIVE To describe the historical trends and compare the characteristics of IMGs to United States medical graduates (USMGs) in the United States. METHODS Longitudinal analysis of the American Medical Association Physicians' Professional Data (AMA-PPD) database using the 1978-2004 files and a comparative analysis of the characteristics of a random sample of 1,000 IMGs and a random sample of 1,000 USMGs using the 2004 file. METHODS Historical trends and characteristics of IMGs in the United States. RESULTS Over the last 26 years, the number of IMGs in the United States grew by 4,873 per year reaching a total of 215,576 in 2004, about 2.4 times its size in 1978. The proportion of IMGs increased 0.12% per year, from 22.2% in 1978 to 25.6% in 2004. In 2004, compared with USMGs, IMGs were older, less likely to be board certified [Odds ratio (OR), 0.68; 95% CI, 0.53 to 0.86], less likely to work in group practice (OR, 0.60; 95% CI, 0.37 to 0.98), more likely to have Internal Medicine as practice specialty (OR, 2.10; 95% CI, 1.62 to 2.71) and more likely to be residents (OR, 1.52; 95% CI, 1.07 to 2.16). CONCLUSIONS Over the last quarter century, the IMGs provided a significant and steady supply for the United States physician workforce that continues to grow. Policymakers should consider the consequences for both the United States and source countries.