This study used cross-sectional data for 19,704 white men and 13,895 white women from the Chicago Heart Association Detection Project in Industry (November 1967 to January 1973) to investigate whether weight explains the association between age and blood pressure, and in particular, whether age is associated with blood pressure and hypertension in the absence of overweight. The relations among age, relative weight, and blood pressure were examined through assessment of mean blood pressure levels and prevalence of hypertension in 25 subgroups stratified by age (18-24, 25-34, 35-44, 45-54, and 55-64 years) and by relative weight (less than 100%, 100-109%, 110-119%, 120-134%, and greater than or equal to 135%). For all five relative weight groups, for both men and women, mean diastolic blood pressure was higher at successive ages. This phenomenon was also seen for systolic blood pressure after ages 35-44 years for men, and after ages 25-34 years for women. On the other hand, the higher the relative weight was, the higher the blood pressure was. Regression analysis demonstrated that the observed relation between age and blood pressure was consistent for all five relative weight groups, including those at desirable weight. These data indicate that for US subjects, age and blood pressure are generally associated in the absence of overweight.