Through its anabolic, lipolytic and antinatriuretic actions, GH has profound effects on body composition. In untreated acromegaly, body weight, body cell mass and extracellular water are increased simultaneously with a decrease of body fat. After successful treatment, extracellular water and body fat normalize, but cell mass remains high. The changes in cell mass, body fat and extracellular water observed in acromegaly suggest different dose-response relationships between GH and these parameters. The relationship between GH concentration and the lipolytic actions of GH is more linear, while the relationship between GH and excess extracellular water is more curvilinear. The sodium-retaining effect of GH seems to be mediated by stimulation of the Na+-K+ pump. At higher GH levels, the pump activity is counteracted by an alleged sodium transport inhibitor. In GH-deficient children, GH treatment is followed by rapid loss of adipose tissue and muscular gain. The influence of GH on body composition in GH-deficient adults has recently received attention. Compared to normal subjects, these patients are overweight and have decreased cell mass. Replacement treatment with GH restores body composition towards normal.