Over a million people reported their race as American Indian in the 2000 U.S. Census but did not report that race in the 1990 Census. We investigate three questions related to this extraordinary population change: (1) Which subgroups of American Indians had the greatest numerical growth? (2) Which subgroups had the greatest proportional increase? And (3) is it plausible that all “new” American Indians reported multiple races in 2000? We use full-count and high-density decennial U.S. census data; adjust for birth, death, and immigration; decompose on age, gender, Latino origin, education, and birth state; and compare the observed American Indian subgroup sizes in 2000 to the sizes expected based on 1990 counts. The largest numerical increases were among non-Latino youth (ages 10-19), non-Latino adult women, and adults with no college degree. Latinos, highly-educated adults, and women have the largest proportionate gains, perhaps indicating that “American Indian” has special appeal in these groups. We also find evidence that a substantial number of new American Indians reported only American Indian race in 2000, rather than a multiple-race response. This research is relevant to social theorists, race scholars, community members, program evaluators, and the Census Bureau.