Calcium uptake from the gut is increased by GH effects on vitamin D metabolism or action. Bone metabolism is stimulated by many factors, of which GH and insulin-like growth factor I (IGF-I) are only two examples. From various animal and human data, it can be shown that GH and IGF-I seem to influence bone formation more than bone resorption. However, GH excess, as seen in acromegaly, does not result in increased bone mass. GH was used to treat osteoporosis in a few clinical trials. In only one, in combination with calcitonin, an increase in bone mass was observed. In GH-defïcient adult patients, substitution therapy resulted in a temporary increase in serum calcium levels. Effects on bone mineral mass were only found in a pilot study performed by us in a group of 8 GH-deficient adult patients. The mean increase of bone mineral mass was 0.04 g hydroxypatite/cm<sup>2</sup>, p < 0.05. These data indicate that GH substitution can result in increased bone mass.