Background: Carolic restriction as a treatment for obesity causes catabolism of body protein stores and produces negative nitrogen balance. GH administration causes acceleration of lipolysis and promotion of nitrogen conservation. We evaluated the effects of GH treatment and caloric restriction on lipolysis, anabolic effects and body composition in obese subjects. Methods: 24 obese(20% over IBW) subjects(22 women and 2 men; 22-46yr old) were fed a diet of 25kcal/kg IBW with 1.2g protein/kg IBW daily during treatment. The subjects were assigned at random to either treatment with recombinent human GH(n=12, 0.06U/kg IBW every other day) or placebo(n 12, vehicle injection) for 12 weeks. Body fat was assessed by impedence and abdominal fat, visceral fat area at the umbilicus level and muscle area of mid thigh level were measured using computed tomography. Results: Fraction of body weight lost as fat lost was significantly greater in GH treatment than in placebo group(1.21+-0.48%/kg, vs 0.52+-0.28%/kg, p0.05). GH treatment caused significant decrease in visceral fat area(35.3% vs 28.5%, p<0.05). In placebo group, there were significant loss of muscle area(-4.8 +- 2.6cm ) and lean body mass(-2.62 +- 1.51kg) after treatment. In contrast, GH treatment group had more increase in muscle area(3.5+-2.3cm ) and lean body mass (1.13 +- 1.04kg) and positive nitrogen balance(1.81+-4.06g/day). GH injections cuased a 1.6-fold increase in IGF-I, despite caloric restriction. GH responses to L-dopa stimulation were blunted in all subjects and GH responses were increased after treatment. Both group showed hyperinsulinemia during oral glucose tolerance test(OGTT), and after treatment, they had decreased in insulin secretion. However, GH treatmnent group had not significant decrease, because GH might induce insulin resistance. FFA response areas during OGTT markedly decreased after treatment in both group. In GH treatment group, more decrease of FFA responses might result from the antilipolytic effect by higher level of insulin or more decrease in amount of fat. Conclusion: This study has demonstrated that in obese subjects fed hypocaloric diet, GH accelerates body fat loss and exerts anabolic effects. ( J Korean Soc Endocrinol 12:571-583, 1997)