Aging is accompanied by involutional changes in testicular function; limited data suggest a decrease in bilateral testicular volume (BTV). We studied BTV by ultrasonography in relation to serum gonadal hormones in 115 healthy elderly men (median age, 78 yr) and 42 young men (median age, 26.5 yr). Elderly men had a clearly smaller BTV (mean, 20.6 vs. 29.7 ml; P < 0.001), whereas serum inhibin B was slightly but significantly decreased (mean, 176.8 vs. 212.8 ng/liter; P = 0.04); lower values in the elderly were observed for bioavailable (Bio) testosterone (T), Bio 17 beta-estradiol, inhibin B/FSH (mean, 18 vs. 58 ng/mU; P < 0.001), and T/LH ratios. In the elderly and the young, respectively, BTV was associated with inhibin B (r = 0.53, P < 0.001; r = 0.41, P < 0.01), FSH (r = -0.53, P < 0.001; r = -0.48, P < 0.01), and inhibin B/FSH ratio. Only in the old men was BTV significantly associated with LH (r = -0.32; P < 0.001), Bio T (r = 0.26; P < 0.01), and T/LH (r = 0.48; P < 0.001). In a multivariate analysis, FSH, inhibin B, and Bio T were independently associated with BTV in the elderly (R(2) = 0.34). Receiver operating characteristics curve analysis indicated that BTV at a criterion value of 14.3 ml had a sensitivity of 46% and a specificity of 79% to predict low serum Bio T levels in the elderly. In conclusion, the moderately decreased BTV observed in elderly men, strongly associated with a decrease of the inhibin B/FSH ratio, is consistent with a reduced Sertoli cell mass, compensated by increased FSH stimulation resulting in only limited decrease of Sertoli cell function. Finding of a low testicular volume in elderly men can contribute to the diagnosis of hypogonadism, but this criterion has low sensitivity to detect decreased T production.