The effect of cryopreservation on the integrity and fertilizing capacity of round spermatids was studied in two azoospermic patients. In December 1995 the patients, both with maturation arrest of spermatogenesis, were submitted to testicular sperm extraction (TESE) after an extensive examination of their ejaculate. Only round spermatids were found after testicular biopsy. Some of the spermatids were isolated and used for a first injection, while the remainder of the preparation was cryopreserved for successive cycles. Because of the failure of the first attempt, 3 months later, the same two patients were submitted to a second one. The frozen preparation was thawed and examined to evaluate the integrity and the viability of surviving round spermatids. More than 70% of the thawed spermatids were viable for injection. Fifteen oocytes at metaphase II, retrieved from the patients' wives, were microinjected with thawed round spermatids. Eighteen hours after the injection, seven out of 15 oocytes showed normal fertilization, with the presence of two pronuclei. The zygotes were cultured to observe embryonic development. After 48 h, six cleaving embryos had developed to at least the two-cell stage, while one had arrested at the pronuclear stage. At 72 h, the cleaving embryos showed further development to the four- to six-cell stage. They were then transferred into the uterus. After 3 weeks a clinical pregnancy was established in one patient [beta-human chorionic gonadotrophin (beta HCG) concentration was 2100 UI]. At 16 weeks of gestation, chromosomic analysis was performed, which confirmed the presence of a fetus with normal karyotype. The pregnancy is ongoing.