In adults of Western societies the positive relationship between blood pressure and body weight has often been demonstrated, both cross-sectionally and longitudinally. This correlation is even stronger in children and early adulthood. In most studies in children, the association between age and blood pressure disappears after controlling for weight. Association must be differentiated from causation. It has however been shown in several intervention studies that treatment of obesity by weight loss decreases blood pressure substantially both in hypertensive and normotensive subjects. Although combining results from several intervention trials is difficult this is the only practical way to get an overall estimate of the hypotensive response to be expected from weight reduction. In the studies presently reviewed, a decrease in weight by 1 kg resulted in a reduction in blood pressure by 3.4/1.3 mm Hg in hypertensive patients and in normotensive subjects the corresponding reductions averaged 1.4 mm Hg and 0.6 mm Hg for systolic and diastolic pressure, respectively.