With no attempt made to influence their diet, six sedentary obese men ages 19 to 31 completed 16 weeks of vigorous walking 90 min, 5 days/week, on a treadmill at up to 3.2 mph on a 10% grade, expending about 1100 kcal per session. Body composition studies indicated a loss of 5.9 kg of body fat and a gain of 0.2 kg of lean tissue for a net loss of 5.7 kg. Percentage body fat decreased from 23.3 to 17.4. Monitored food intake initially increased, then progressively decreased below pretraining levels. Work capacity and cardiovascular efficiency improved with training. Plasma cholesterol and triglyceride concentrations were not significantly changed; however, high density lipoprotein cholesterol progressively increased to 15.6% above pretraining levels and the high/low density lipoprotein ratio increased 25.9%. Fasting blood sugar was significantly lower after training. Blood glucose concentrations after a glucose challenge did not significantly change, but a 43% reduction in plasma radioimmunoassay insulin levels and a 36% decrease in the ratio of insulin/glucose concentration occurred. Thus, vigorous regular walking resulted in a reduction of body fate stores, endogenous insulin requirements, and food intake, and perhaps improved the ability to eliminate cholestrol by increasing the plasma high density lipoprotein fraction.