The objective of this study was to explore the relationship between the severity of oligozoospermia and the development of embryos and clinical outcome in patients undergoing ICSI. A total of 908 intracytoplasmic sperm injection cycles involving women of < or =37 years of age were included in this study. The patients were divided into four treatment groups according to the results of an analysis of their husbands' semen: (A) mild oligozoospermia that ranged from 10 x 10(6)/ml to <20 x 10(6)/ml (n=283), (B) mild to severe oligozoospermia that ranged from 5 x 10(6)/ml to <10 x 10(6)/ml (n=192), (C) severe oligospermia that ranged from 1 x10(6)/ml to <5 x 10(6)/ml (n=259), and (D) very severe oligozoospermia that ranged from 0 to <1 x10(6)/ml (n=174). Two pronuclei (PN) oocytes at MII were injected and the development of high quality embryos on day 2, as well blastocyst formation rate on day 5, the implantation rate, clinical pregnancies, and fetal loss, were examined. A lower percentage of two pronuclei (2PN) oocytes in the very severe oligozoospermia group was observed, however, there was no difference in clinical outcome when the oligozoospermic patients were divided by sperm concentration. In addition, no significant difference was detected in zygote production or clinical outcome between spermatozoa with a motility of <40% and spermatozoa with a motility of > or =40%. The results of this study emphasize the importance of selecting good quality sperm for oocyte injection, especially in cases involving very severe oligozoospermia.