This article examines racial classifications on United States population census schedules between 1890 and 1990 to provide insights on the changing meanings of race in US society. The analysis uses a sociology of knowledge perspective which assumes that race is an ideological concept that can be interpreted most productively by relating its definition and measurement to the larger social and political context. Four themes are identified and discussed: (i) the historical and continuing importance of skin colour, usually dichotomized into white and non-white, in defining race and counting racial groups; (ii) a belief in ‘pure’ races that is reflected in a preoccupation with categorizing people into a single or ‘pure’ race; (iii) the role of census categories in creating pan-ethnic racial groups; and, (iv) the confusing of race and ethnicity in census racial classifications. Each theme demonstrates the potential or actual role of official statistics, exemplified by census racial data, in reflecting and guiding changes to the meaning and social perceptions of race. A detailed examination of racial classifications from the 1980 and 1990 Censuses shows that the influence of political interests on racial statistics is particularly important. The article concludes with a discussion of whether official statistical recorders such as population censuses should categorize and measure race, given the political motivations and non-scientific character of the classifications used.