A longitudinal study of the effectiveness of the "Know Your Body" (KYB) program in reducing coronary heart disease risk factors was begun among black students in the District of Columbia in 1983. Subjects were in grades four through six at nine schools stratified on socioeconomic status and randomized into one control and two intervention groups. At baseline, 1,041 students were measured for systolic and diastolic blood pressure, ponderosity, triceps skinfold thickness, postexercise pulse recovery rate, serum thiocyanate, serum total cholesterol, and serum HDL cholesterol. Significant net changes in individual values occurred in the favorable direction at one or all four annual follow-up reexaminations for systolic blood pressure, diastolic blood pressure, HDL cholesterol, HDL/total cholesterol ratio, serum thiocyanate, and fitness. Favorable changes in diastolic blood pressure and serum thiocyanate were observed at all reexaminations, and these were substantiated by analyses that used the school grade as the unit of analysis. Intervention students who were judged to have had the best KYB teachers showed significant favorable net changes in total serum cholesterol after one year. Results are consistent with other evaluations of the Know Your Body program suggesting that KYB may reduce chronic disease risk in diverse school populations, and that increased efforts should be made to improve implementation methods.