Osteoporosis in elderly men as well as women is increasingly recognized, and patients with type 2 diabetes mellitus have higher risk of fracture than nondiabetic subjects. The aim of the present study was to investigate the relationship between bone stiffness and serum testosterone concentration as well as other variables in men with type 2 diabetes mellitus. The relationships between bone stiffness and serum bioavailable testosterone concentrations as well as other variables including age, duration of diabetes, glycemic control (hemoglobin A(1c)), or body mass index were evaluated in 294 men with type 2 diabetes mellitus. An inverse correlation was found between stiffness index and age. A positive correlation was found between stiffness index and serum bioavailable testosterone concentration (r = 0.231, P = .0005). Stiffness index was significantly less in current smokers (81.6 +/- 17.7) than in past smokers (86.6 +/- 17.8, P = .0396) or nonsmokers (87.7 +/- 15.2, P = .0426). Multiple regression analysis demonstrated that serum bioavailable testosterone concentration (beta = .271, P = .0006) and smoking status (beta = -0.147, P = .0408) were independent determinants of stiffness index. In conclusion, bone stiffness was associated with serum bioavailable testosterone concentration but not associated with hemoglobin A(1c) or duration of diabetes in men with type 2 diabetes mellitus.