Serum estradiol levels were measured in 360 men attending an infertility clinic and 68 proven fertile men to determine whether estradiol measurements are clinically useful. The normal range of estradiol levels found in fertile men was 10-82 pg/ml. Serum concentrations of estradiol in azoospermic or oligozoospermic patients were significantly lower than those in normozoospermic men (p < 0.021). Serum concentrations of testosterone were also significantly decreased in infertile patients (p < 0.025). This decrease in serum estradiol may be partly due to reduced testosterone levels in these men because estradiol is mainly formed by peripheral aromatization of testosterone in fatty and muscle tissues. However, the exact mechanism for the decrease in the serum estradiol levels remains undetermined. It is concluded that serum estradiol levels are significantly low in the men with various testicular disorders.