Measurement of muscle and fat tissue mass by magnetic resonance imaging of the thighs was used to study the metabolic effect of human (h) GH in 23 GH deficiency, 8 Turner's syndrome, and 14 intrauterine growth retardation prepubertal patients. They were evaluated before and 1, 3, 6, and 12 months after the onset of hGH treatment. Seven normal children were followed longitudinally as controls. During hGH treatment, a significant increase in muscle tissue and decrease in adipose tissue cross-sectional areas was observed, leading to a dramatic increase in the muscle/adipose tissue cross-sectional area ratio in each period studied. These findings remained highly significant when corrected for the small variation observed in controls. The body mass index was correlated with muscle and adipose tissue cross-sectional area at each time point (P < 0.0001). The muscle cross-sectional area increment correlated with the first year height velocity (P < 0.01). This study indicates that in children with and without GH deficiency, hGH therapy induces rapid and intense variation of muscle and adipose tissue mass, and that magnetic resonance imaging can be used to study some aspects of the metabolic actions of GH.