Children with isolated GH deficiency have enlarged fat depots due to increased fat cell volume. The fat cell number is however decreased compared with appropriate controls. Treatment of patients with GH results in a relative loss of body fat and shifts both fat cell number and fat cell volume towards normal. These findings of adipose tissue cellularity in GH-deficient children before and after treatment can be explained by results from in vitro studies indicating that GH exerts a variety of effects on the metabolism of mature fat cells and on the proliferation and differentiation of cultured preadipocytes. Therefore, GH is one of the factors that determines adipose tissue development during childhood.