We determined spermatogenic patterns of seminiferous tubules in azoospermic infertile men and evaluated the prevalence of bilateral testicular homogeneity. 185 azoospermic men underwent bilateral testicular fine-needle aspiration (TFNA) in which each testis was punctured at 3 different positions. Aspirated material was stained and classified according to the most mature spermatogenic cell type present or whether only Sertoli cells were present. 35.7% had spermatozoa in their testes, 36.2% had spermatogenic maturation arrest, and 28.1% had only Sertoli cells in their seminiferous tubules. In 15.6% of all patients, the diagnosis in 1 testis differed from that in the other. In only 73.2% of those with testicular spermatozoa was it bilateral. In the remaining 26.9%, only Sertoli cells, spermatocytes or spermatids were found as the most mature cell type in the other testis. The study definitely indicates that fertilization with retrieved testicular spermatozoa should not be offered to azoospermic patients without prior evaluation of the seminiferous tubuespermatogenic pattern in both testes.