Obesity and physical inactivity are associated with both elevated cardiovascular risk and blood pressure (BP), but the interrelation of exercise, weight loss and BP is poorly understood. This study examines the independent effects of exercise and weight loss on both standard clinic and automated, ambulatory BP in 115 overweight, sedentary, normotensive men (aged 30 to 59 years) who were randomly assigned to control status or to lose weight over 1 year by moderate caloric restriction (dieting) or by increased caloric expenditure (exercise). Median daytime and evening BP were determined from measurements made every 20 minutes while the subjects were awake. After 1 year, the control group gained (mean +/- standard deviation) 0.5 +/- 3.8 kg while the diet group lost 6.9 +/- 4.4 kg and the exercise group lost 4.6 +/- 3.5 kg. Clinic BP decreased similarly in all 3 groups, but daytime and evening ambulatory BP decreased in both intervention groups and increased in the control group. Relative to the 1-year change in control subjects, net change in daytime ambulatory BP averaged -2 to -3 mm Hg in both dieters and exercisers, while net change in evening ambulatory BP averaged -3 to -4 mm Hg. These changes were all statistically significant (p less than 0.05) when compared with control subjects except for daytime systolic BP in both intervention groups and evening diastolic BP in dieters. Weight loss achieved through caloric restriction or expenditure may cause important decreases in BP in normotensive men; exercise appears to confer no unique benefit. If confirmed, these results have important public health implications for the prevention of cardiovascular disease.