Spermatogenesis was investigated in seminiferous tubules of 100 males with nonobstructive azoospermia. Forty-four (44%) cases had Sertoli-cell-only syndrome, 23 (23%) had spermatogonium in the tubules, 17 (17%) had primary spermatocyte in the tubules, and 16 (16%) had round or late spermatid in the tubules. No cases showed secondary spermatocyte present in the tubules. The mean serum follicle-stimulating hormone (FSH) in males with nonobstructive azoospermia was significantly higher than that in males with obstructive azoospermia (p <. 001). The mean concentrations of serum FSH in cases with Sertoli-cell-only syndrome and spermatogonium in the tubules were significantly higher than those in cases with primary spermatocyte and spermatid in the tubules (p <. 05-.001). The results indicate that the evaluation of testicular histology using the type of germ cells present in seminiferous tubules is available for assisted reproductive technology.