It is unknown whether loss in musculo-tendinous tissue during inactivity can be counteracted by growth hormone(GH), and whether GH accelerate rehabilitation in aging individuals. Elderly men (65-75 years; n=12) had one leg immobilized two weeks followed by six weeks re-training, and were randomly assigned to daily injections of recombinant GH(rhGH, n=6) or placebo(Plc, n=6). Cross sectional area(CSA), muscle strength(MVC) and biomechanical properties of m. quadriceps and patellar tendon were determined. Muscle and tendon biopsies were analyzed for gene expressions (mRNA) of collagen(COL1A1/3A1) and insulin-like growth factors(IGF-1Ea/Ec). Fibril morphology was analyzed by transmission electron microscope(TEM). In tendon, CSA and biomechanical properties did not change following immobilization, but an increase in CSA was found after 6 weeks of rehabilitation in both groups. The changes were more pronounced when GH was injected. Furthermore, tendon stiffness increased in GH group. Muscle CSA declined after immobilization in Plc but not in GH group. Muscle CSA increased during re-training with a significant larger increase in GH group compared to Plc. Both a time and group effect was seen for IGF-1Ea/Ec and COL1A1/3A1 mRNA expression in muscle with a difference between GH and Plc. IGF-1Ea/Ec and COL-1A1/3A1 mRNA expression increased in muscle following immobilization and re-training in subjects receiving GH, whereas an increase in IGF-1Ec mRNA expression was seen in the Plc group only after re-training. In conclusion, in elderly humans GH seems to have a matrix stabilizing effect during inactivity and rehabilitation by stimulating collagen expression in the musculo-tendinous tissue and increasing tendon CSA and stiffness.