Background: The growth hormone deficient adults have reduced lean body mass and increased fat mass. And growth hormone deficiency leads to other endocrinologic and metabolic alterations. In recent studies, growth hormone replacement therapy improved these endocrinologic and metabolic alterations. Methods: In a placebo controlled trial, we studied the effects of six months treatment of recombinant human growth hormone (rhGH) in 28 adults with GH deficiency, The patients had acquired GH deficiency due to pituitary tumors (13 cases), Sheehan`s syndrome (13 cases), and head trauma (2 cases). They were randomly divided into three groups. The group 1 (10 cases, M:F= 5:5) received 0.06 U of rhGH per kilogram of body weight subcutaneously three times a week at bed time, the group 2 (9 cases, M:F=4:5) received same amout of rhGH daily, and the group 3 (9 cases, M:F=4:5) received placebo. Results; In all three groups the total body weight did not change, whereas lean body mass significantly increased and adipose tissue mass decreased in rhGH treatment groups. The plasma glucose and glycated hemoglobin levels were not increased during rhGH treatment. The bone mineral densities of spines and femurs were not changed. According to computed tomography on two levels (umbilicus and mid-thigh), the mean mid-thigh muscle mass was significantly increased in group 1, while visceral fat mass significantly decreased in group 2. In groups 1 and 2, rhGH slightly increased exercise maximum oxygen uptake, significantly increased exercise rate-pressure product, hand grip strength and back lift strength after rhGH treatment, but the placebo group had not any change. Conclusion: These results suggest that rhGH has various potentially beneficial effects in GH-deficient adults, especially in changing body composition and increasing exercise capacity.