BACKGROUND: Hypopituitarism is associated with osteoporosis and osteopenia especially when hypogonadotropic hypogonadism is present. Despite hypopituitarism being an important cause of secondary osteoporosis, osteoporosis in patients receiving surgery for pituitary tumors in Korea has not been studied. In this study, we evaluated the effects of testosterone replacement therapy (TRT) on bone mineral density (BMD) in postoperative hypogonadal patients with pituitary tumors. METHODS: To examine the effect of TRT on BMD, we performed a retrospective observational study in 21 postoperative male patients who underwent pituitary tumor surgery between 2003 and 2012 at the Ajou University Hospital. Testosterone was replaced in postoperative hypogonadal patients by regular intramuscular injection, daily oral medication, or application of transdermal gel. BMD (g/cm2) measurements of central skeletal sites (lumbar spine, femoral neck, and total femur) were obtained using dual-energy X-ray absorptiometry (GE Lunar). For lumbar spine BMD, L1 to L4 values were chosen for analysis. Femur neck and total femur were also analyzed. RESULTS: During the follow-up period (mean, 56 months; range, 12 to 99 months) serum testosterone levels increased with the administration of TRT (P=0.007). There was significant improvement (4.56%+/-9.81%) in the lumbar spine BMD compared to baseline BMD. There were no significant changes in the femur neck BMD or total femur BMD. We did not find any statistically significant relationships between changes in testosterone levels and BMD using Spearman correlation analysis. CONCLUSION: Our results indicated that TRT used in the postoperative period for hypogonadal pituitary tumor surgery patients may have beneficial effects on the BMD of the spine.