No research has been done on the body mass index values of the 19th-century Mexican population. This paper introduces a new data source of 19th-century Mexican male inmates in American prisons and finds that the majority of Mexican body mass was in the normal range (18.5–24.9). Few Mexicans were underweight or obese by modern standards. Body mass varied little (0–0.9units) by occupations possibly because criminals probably came from the lower end of the socioeconomic distribution; however, it did vary with crimes for which inmates were incarcerated.