All tenth graders in four senior high schools (N = 1447) from two school districts participated in a cardiovascular disease risk-reduction trial. Within each district, one school was assigned at random to receive a special 20-session risk-reduction intervention and one school served as a control. At a two-month follow-up, risk factor knowledge scores were significantly greater for students in the treatment group. Compared with controls, a higher proportion of those in the treatment group who were not exercising regularly at baseline reported regular exercise at follow-up. Almost twice as many baseline experimental smokers in the treatment group reported quitting at follow-up, while only 5.6% of baseline experimental smokers in the treatment group graduated to regular smoking compared with 10.3% in the control group. Students in the treatment group were more likely to report that they would choose "heart-healthy" snack items. Beneficial treatment effects were observed for resting heart rate, body mass index, triceps skin fold thickness, and subscapular skin fold thickness. The results suggest that it is feasible to provide cardiovascular disease risk-reduction training to a large segment of the population through school-based primary prevention approaches.