We examined the effects of biosynthetic growth hormone (GH) on biochemical indices of bone turnover and on bone mineral density in a group of GH-deficient adults. Thirteen patients (eight males and five females) aged 24 +/- 5 years (range 16-35) were studied before and 12 and 24 months after GH treatment (0.1 IU.kg-1 day-1, 6 days a week). Serum levels of insulin-like growth factor I (IGF-I), calcitonin, parathyroid hormone, alkaline phosphatase, intact osteocalcin, fasting urinary hydroxyproline/creatinine ratio and bone mineral density (BMD), measured at the lumbar spine by dual-photon absorptiometry, were evaluated. After 12 months of treatment, IGF-I, alkaline phosphatase, osteocalcin and the fasting urinary hydroxyproline/creatinine ratio increased significantly. However, after 24 months of therapy, serum levels of osteocalcin decreased to pretreatment values while IGF-I, fasting urinary hydroxyproline/creatinine ratio and alkaline phosphatase remained elevated significantly. No changes were found in parathyroid hormone and calcitonin plasma levels or in BMD either after 12 or 24 months of treatment. These data demonstrate that GH, at the dosage that we used, activates bone turnover during 24 months of therapy in adults with panhypopituitarism, even if a downward trend for osteocalcin became apparent at 24 months. However, this activation in bone turnover was not accompanied by an increase in BMD. We can hypothesize that GH, at the relatively high dosage used, may stimulate osteoclastic activity to a greater extent than osteoblastic activity. It is probable that the dose of GH replacement therapy in adults plays a key role.