The effect on bone, protein, carbohydrate and lipid homeostasis as well as body composition of the administration of growth hormone to adult patients with growth hormone deficiency was studied. Growth hormone was administered at a dose of 25 micrograms/kg/day with a maximum of 1.48 mg (4 IU) a day, for 6 months in eight adults. Studies were done before the start and at 1, 3 and 6 months during therapy, as well as 3 months after treatment had been stopped. Subjective well-being as assessed by a short psychological tests showed an improvement in six and no change in two patients. Body composition, as assessed by body impedance assessment and D2O dilution, both showed an increase in lean body mass of 4 kg (5% of body weight), accompanied by a decrease in mean fat mass of 3 kg. Nitrogen turnover studies showed a transient increase in fed state nitrogen balance due to an increase in the rate of protein synthesis, which exceeded a smaller increase in the protein degradation rate. Growth hormone treatment did not affect the circulating levels of 25(OH)-vitD or PTH 1-84, while 1,25(OH)2-vitD had significantly increased after 6 months, as well as 3 months after treatment ended. Osteocalcin, procollagen I levels, as well as 24-hour urinary excretion of hydroxyproline and calcium rose during growth hormone administration but subsequently decreased rapidly after administration had been stopped, while the increase in alkaline phosphatase persisted. This increase in markers of both bone resorption and bone formation indicates an activation of bone remodelling, but this was not reflected by an increase in bone density. Glucose levels, measured before and during a normal breakfast, increased during growth hormone treatment, but serum insulin levels did not. Total cholesterol levels decreased by 0.5 mmol/l. Levels of T4 and free T4 as well as rT3 decreased, while T3 increased during growth hormone treatment. Therapy with growth hormone for 6 months in a dose varying between 6 and 25 micrograms/kg/day increased lean body mass and decreased fat mass. The sense of general well-being improved in most patients. Furthermore, growth hormone treatment increased bone turnover without a measurable increase in bone density, caused some minor changes in lipid and carbohydrate metabolism, and increased the metabolism of thyroxine to T3.