Treatment with beta 2-agonists promotes fat loss and muscle growth in numerous species, but human studies are lacking. We studied the effect of a compound with beta 2-agonistic properties (ephedrine 20 mg/caffeine 200 mg [E + C]). Fourteen obese women were treated with a 4.2-MJ/d diet and either E + C or placebo (P) three times per day for 8 weeks in a double-blind study. Weight-loss was not different in the groups, but the E + C group lost 4.5 kg more body fat and 2.8 kg less fat-free mass (FFM). The decrease in 24-hour energy expenditure (EE) seen in the P group was 10% at day 1 and 13% at day 56, but was only 7% and 8% in the treated group (P = .044). The higher EE in the E +C group was entirely covered by fat oxidation. These findings provide evidence that promotion of fat loss and preservation of FFM during weight reduction may also be achieved pharmacologically in humans.