Although a specific GH deficiency (GHD) syndrome in the adult and the response to GH replacement therapy are well recognized, there are few data available on the effect of GH replacement therapy in elderly GH-deficient patients. We studied the effect of GH therapy on body composition and bone mineral density measured by dual energy x-ray absorptiometry, markers for bone metabolism, insulin-like growth factors (IGFs), and IGF-binding proteins (IGFBPs) in 31 patients (6 women and 25 men; aged 60-79 yr; mean, 68 yr) with multiple pituitary hormone deficiencies. The GH response to arginine or insulin was below 3 microg/L (9 mU/L) in all subjects. They were randomized to GH (Humatrope, Eli Lilly & Co.) or placebo for 6 months, followed by 12 months of open treatment. The dose was 0.05 IU/kg x week for 1 month, and after that it was 0.1 IU/kg x week divided into daily sc injections (0.75-1.25 IU/day). There were no changes in any of the measured variables during placebo treatment. GH treatment normalized serum IGF-I in a majority of the patients and increased IGFBP-3 and -5 as well as IGFBP-4 and IGF-II to values within normal range. Lean body mass was increased, and the increase at 6 and 12 months correlated with the increase in IGF-I (r = 0.46; P = 0.010 and r = 0.54, respectively; P = 0.003). GH treatment caused a modest, but highly significant, reduction of total body fat. Mean bone mineral density was not different from that in healthy subjects of the same age and did not change during the observation period. Markers for bone formation (bone-specific alkaline phosphatase activity, osteocalcin, and procollagen I carboxyl-terminal peptide in serum) increased within the normal range, and levels were sustained throughout the study. The bone resorption marker (pyridinoline in urine) was significantly elevated for 12 months. Side-effects were mild, mostly attributed to fluid retention. In two patients with normal glucose tolerance at the start of the study, pathological glucose tolerance occurred in one patient and was impaired in one. In conclusion, elderly patients with GHD respond to replacement therapy in a similar manner as younger subjects, with an improvement in body composition and an increase in markers for bone metabolism. Side-effects are few, and elderly GHD patients can be offered treatment. As long-term risks are unknown, GH doses should be titrated to keep IGF-I within the age-related physiological range.