Purpose: Klinefelter syndrome (KS) is related to testicular insufficiency, which causes low testosterone levels in serum. Generally, sex hormone levels and bone mineral density (BMD) are lower in patients with KS than normal. We investigated the effects of testosterone replacement on serum testosterone levels and BMD in KS patients. Materials and Methods: From December 2005 to March 2008, 18 KS patients with a 47, XXY karyotype were treated with initial intramuscular injections of long-acting testosterone undecanoate (Nebido (R), 1000 mg/4 mL) at baseline and second injections after six weeks. An additional four injections were administered at intervals of 12 weeks after the second injection. BMD was measured at the lumbar spine (L2-4), the left femoral neck and Ward's triangle, using dual energy X-ray absorptiometry. Medical histories, physical examinations and prostate specific antigen, hematology and serum chemistry were conducted for each patient. In addition, total testosterone and sex hormone-binding globulin levels were measured. Results: Following testosterone replacement, mean serum total testosterone increased significantly from baseline (0.90 vs. 4.51 ng/mL, p<0.001), and total testosterone rose to normal levels after replacement in all patients. The mean BMD of the lumbar spine increased significantly (0.91 vs. 0.97 g/cm(2), p<0.001). Similar increases of BMD were also observed at the femoral neck, but this increase was not significant. Conclusion: These findings suggest that testosterone replacement therapy may be effective in treating BMD deficiency in men with testosterone deficiency, especially those with Klinefelter syndrome.