The aim of the present study was to evaluate relationships between serum endogenous androgens and urinary concentration of cross-linked N-telopeptides of type I collagen (NTx), a bone resorption marker, in men with type 2 diabetes mellitus because low androgen concentrations are associated with both osteoporosis and cardiovascular disease. Relationships between serum free testosterone and urinary NTx concentrations were investigated in 246 consecutive men with type 2 diabetes mellitus. In addition, relationships between urinary NTx concentration and other variables including age, duration of diabetes, blood pressure, serum lipid concentration, hemoglobin A(1c), and body mass index were evaluated. Urinary NTx concentrations were 27.8 (26.4-29.3) nmol of bone collagen equivalent per millimole of creatinine, correlating inversely with serum free testosterone (r = -0.263, P < .0001). Multiple regression analysis identified serum free testosterone (beta = -.292, P < .0001), hemoglobin A(1c) (beta = .144, P = .0404), and smoking status (beta = .143, P = .0402) as independent determinants of urinary NTx. In conclusion, serum free testosterone concentration correlated inversely with urinary NTx concentration, which may partly account for an observed link between osteoporosis and cardiovascular disease in men with type 2 diabetes mellitus.