More than a million people reported their race as American Indian/Alaska Native (AIAN) in the 2000 U.S. Census but did not do so in the 1990 Census. We ask: (1) Which subgroups had the greatest numerical growth? (2) Which subgroups had the greatest proportional increase? And (3) are the 2000 single-race AIANs and the 1990 AIANs the same set of people? We use full-count and high-density decennial census data; adjust for birth, death, and immigration; decompose on age, gender, Latino origin, education, and birth state; and compare the observed subgroup sizes in 2000 to the sizes expected based on 1990 counts. The largest numerical increases were among adolescent and middle-aged non-Latinos, non-Latino women, and adults with no college degree. Latinos, women, highly-educated adults, and people born in Eastern states had the largest proportionate gains. The ability to report multiple races in 2000 and the new federal definition of “American Indian” may have especially affected these groups, though personal identity changes are probably also involved. We find that thousands of new Latino AIANs reported only one race in 2000. Also, many 1990 AIANs reported multiple races in 2000. Thus, the 1990 AIANs and 2000 single-race AIANs are not always the same individuals.