OBJECTIVE Although skeletal muscle wasting can occur in chronic kidney diseases, its relationship with the serum testosterone concentration remains uncertain. This study investigates the relationship between serum testosterone and skeletal muscle mass in men under hemodialysis (HD). METHODS Sixty men aged between 41 and 89 years undergoing HD for 15.0 ± 8.1 years were enrolled for this study. The muscle areas of the thigh (TMA) and abdomen (AMA) were measured by computed tomography (CT), and the association between these muscle areas and serum total testosterone was examined with adjustment of age and other nutritional variables. RESULTS The mean serum total testosterone in our HD patients (6.33 ± 2.90 ng/mL) was not lower than that of the Japanese general population, but showed a positive correlation with TMA (r = 0.39, p < 0.05), AMA (r = 0.52, p < 0.001), serum creatinine (r = 0.33, p < 0.05), and the creatinine generation rate (r = 0.26, p < 0.05). Serum total testosterone was inversely correlated with age (r = -0.32, p < 0.05), CRP (r = -0.31, p < 0.05), and IL-6 (r = -0.24, p < 0.05). A multiple-regression analysis showed both serum total testosterone and age to be an independent determinant of the muscle mass in these patients. CONCLUSIONS This study identified testosterone as a determinant of muscle mass in HD men.