Clinical interaction of salt and weight change on blood pressure level. Studies that examine the effect of altered body weight or dietary sodium on blood pressure level are reviewed. Emphasis is placed on studies that compare the effects of weight reduction or sodium restriction in separate comparison groups or analyze the independent effect of the two intervention modalities in multiple regression analysis. Additional analyses of the Hypertension Prevention Trial data are presented. Most studies, where comparison can be made, suggest a greater effect for weight reduction than for the achieved, moderate, or short-term reduction in sodium intake on subsequent lowering of blood pressure; the lowering effect on blood pressure of weight reduction is independent of the effect of dietary sodium restriction. However, long-term (over a year) substantial reductions (up to 70 meq/day) of dietary sodium have not been achieved in comparative trials, nor are the effects of combined weight and sodium reduction in the prevention and treatment of hypertension quantified. Future studies and potential problems are discussed. 