Previous research shows that people commonly exaggerate the size of minority populations. Theories of intergroup threat predict that the larger people perceive minority groups to be, the less favorably they feel toward them. We investigate whether correcting Americans’ misperceptions about one such population—immigrants—affects related attitudes. We confirm that non-Hispanic Americans overestimate the percentage of the population that is foreign-born or in the United States without authorization. However, in seven separate survey experiments over 11 years, we find that providing accurate information does little to affect attitudes toward immigration, even though it does reduce the perceived size of the foreign-born population. This is true even when people’s misperceptions are explicitly corrected. These results call into question a potential cognitive mechanism that could underpin intergroup threat theory. Misperceptions about the size of minority groups may be a consequence, rather than a cause, of attitudes toward those groups.