Medical data from Selective Service registrants born from 1939 to 1941 were studied in Oregon and Colorado. Among 29,119 registrants with medical information (41.3% of all registrants), 1,073 (3.6%) had definite hypertension, and 1,226 (4.2%) had some type of color blindness. In both states, there was a highly significant association between the prevalence of hypertension and the prevalence of color blindness. Thus, definite hypertension was present in 6.0% of colorblind individuals but in only 3.6% of those with unimpaired color vision, while color blindness occurred in 6.8% with definite hypertension, in 5.8% with borderline hypertension, and in only 4.0% with normal blood pressure. The data did not differentiate among types of color blindness, and the reason for the association is not yet evident.