The foreign-born represent a disproportionate share of nonelderly U.S. adults without health insurance. Using data from Los Angeles County, we find that most of the insurance disparities between the foreign-born and native-born can be explained by traditional socioeconomic factors. Undocumented immigrants, however, have lower rates of coverage-both private and public-even after a wide array of factors are controlled for. Applying Los Angeles County rates to the U.S. population implies that undocumented immigrants account for one-third of the total increase in the number of uninsured adults in the United States between 1980 and 2000.