To clarify the role of sodium intake in the regulation of blood pressure in obese subjects, we measured blood pressure in 60 obese and 18 nonobese adolescents after successive two-week periods of a high-salt diet (greater than 250 mmol of sodium per day) and a low-salt diet (less than 30 mmol per day). When they were changed from a high-salt to a low-salt diet, the obese group had a significantly larger mean change (+/- SE) in mean arterial pressure (-12 +/- 1 mm Hg) than did the nonobese group (+1 +/- 2 mm Hg; P less than 0.001). The variables that best predicted the degree of sodium sensitivity were the fasting plasma insulin level, the plasma aldosterone level while the low-salt diet was being given, the plasma norepinephrine level while the high-salt diet was being given, and the percentage of body weight made up by fat. Fifty-one of the obese adolescents were also studied before and after a 20-week weight-loss program. After the weight-loss program, the 36 subjects who lost more than 1 kg of body weight had a reduced sensitivity of blood pressure to sodium (difference from value during high-salt diet to that during low-salt diet, -1 +/- 1 mm Hg). The blood pressure of the remaining 15 adolescents was still sensitive to sodium intake (-11 +/- 3 mm Hg). These results support the hypothesis that the blood pressure of obese adolescents is sensitive to dietary sodium intake and that this sensitivity may be due to the combined effects of the hyperinsulinemia, hyperaldosteronism, and increased activity of the sympathetic nervous system that are characteristic of obesity.