Background. Questions regarding the efficacy of nonpharmacologic approaches for the treatment of stage 1 hypertension were addressed as part of the Treatment of Mild Hypertension Study (TOMHS), a 4-year, randomized clinical trial (N = 902). This report describes the lifestyle intervention program used in TOMHS, presents data on the lifestyle changes observed, and focuses on the effect of weight loss on blood pressure and blood lipid levels. Methods. Participants were randomly assigned to receive either placebo or one of five different antihypertensive medications. All took part in a lifestyle intervention program to reduce weight and sodium and alcohol intake and to increase physical activity. Results. Substantial changes from baseline levels were achieved for all lifestyle intervention variables. Mean weight change was -10.5 lb (-5.6%) at 1 year, -8.5 lb (-4.5%) at 2 years, -7.4 lb (-4.0%) at 3 years, and -5.7 lb (-3.0%) at 4 years. At 4 years, 70% of participants remained below baseline weight and 34% maintained a weight loss of 10 lb or greater. Mean change in urinary sodium excretion was -12.5 mmol/8 hr (-23%) at 1 year, -10.7 mmol/8 hr (-20%) at 2 years, -8.4 mmol/8 hr (-16%) at 3 years, and -4.6 mmol/8 hr (-9%) at 4 years. Alcohol intake declined by 1.6 drinks/week among drinkers at 4 years. Reported leisure physical activity increased by 86% at 1 year and remained 50% above baseline at 4 years. Beneficial changes in blood pressure and serum lipids were associated with these changes. Conclusions. These results support a role for lifestyle interventions as the initial treatment for stage 1 hypertension and demonstrate that such interventions can be successfully implemented in the clinical setting.