The role of insulin in the regulation of blood pressure was evaluated in 50 obese adolescents before and after a 20-week weight loss program. When compared with 10 nonobese adolescents, the obese subjects had significantly higher systolic, diastolic, and mean arterial pressures (p = 0.005), an elevated 24-hour urinary sodium excretion (p = 0.002), an elevated fasting insulin concentration (p = 0.001), and an abnormal insulin response to an oral glucose tolerance test (sum of the insulins at 0, 1, and 2 hours post-oral glucose load; p = 0.001). We also observed a significant correlation between systolic and diastolic blood pressure (age and sex normalized) and body weight (r = 0.57, p less than 0.01 and r = 0.7, p less than 0.01), fasting insulin (r = 0.49, p less than 0.01 and r = 0.54, p less than 0.01), and sum of insulins (r = 0.42, p less than 0.01 and r = 0.46, p less than 0.01). To study the effect of weight loss on the relationship between blood pressure and insulin, the obese subjects were randomly assigned to three groups: 15 to a diet and behavior change group, 18 to a diet, behavior change, and exercise group, and 17 to an obese control group. Compared with the obese control group, the two weight loss groups each experienced a significant decrease in insulin (p less than 0.01), sum of the insulins (p less than 0.01), and blood pressure (p less than 0.01). The decrease in blood pressure during the weight loss program significantly correlated with the change in both insulin and body weight.(ABSTRACT TRUNCATED AT 250 WORDS)