The beneficial effect of habitual physical exercise, and physical fitness, on blood pressure has been reported in the past. However, the reported fall in blood pressure, associated with physical exercise, could be related to at least one confounding factor. That is, the independent effect of weight loss caused by physical exercise, on blood pressure. Exercise results in loss of body fat, a redistribution of fat stores, and weight loss. All of these are associated with a concomitant reduction in blood pressure. It is not clear whether the reduction in blood pressure is caused independently by a decrease in caloric intake alone, or an increase in physical activity, or the combination of the two. This is particularly true in the case of primary prevention of hypertension in the community where an effective and acceptable strategy for life-style modification has to be developed; a strategy which must be based on valid scientific evidence. The Trial of Hypertension Prevention (TOHP), a large scale randomized clinical trial, was designed to test the hypothesis on the efficacy of weight loss (induced by a combination of reductions in caloric intake and increased physical exercise) on primary prevention of hypertension. Eligible participants were randomized into either the weight loss group, or the control group. The exercise component of the weight loss program included mild exercise, 3-4 days a week for 35-40 minutes at an intensity of 40-45% of heart rate reserve. The dietary component included a reduction in caloric intake to less than 1200 KCal per day. After 18 months, the difference in weight loss between the two groups was 3.9 Kg in favor of the dietary modification and exercise (P < 0.01). This was accompanied with a significant reduction in blood pressure (P < 0.01). Weight loss was a significant predictor for blood pressure change over time (P < 0.001). These findings confirm the efficacy of maintaining an ideal body weight, by a program which combines physical exercise and reduction in caloric intake, on primary prevention of hypertension in the community.