We present the first evidence that genetically damaged human spermatozoa are able to form normal pronuclei in oocytes after intracytoplasmic sperm injection (ICSI). The role of reactive oxygen species (ROS) as a cause of chromatin and DNA damage is well recognized. The same class of molecule can be found in the semen of males with severe infertility, who remained infertile until the advent of ICSI. In this study we have investigated the role of ROS in the induction of chromatin damage, DNA strand breakage and the subsequent ability of spermatozoa to decondense and form pronuclei after ICSI. Spermatozoa from normozoospermic men participating in our research programme were exposed to oxidizing environments created by co-incubation with hydrogen peroxide, reduced nicotinamide adenine dinucleotide phosphate (NADPH) or activated white cells. The subsequent ability of the spermatozoa to decondense in vitro was examined using sequential incubations in EDTA, dithiothreitol and sodium dodecyl sulphate, and the amounts of DNA strand breakage were assessed using an in-situ nick translation protocol. Finally, cells exposed to hydrogen peroxide, NADPH and activated leukocytes were microinjected into hamster oocytes, and their ability to decondense and form normal pronuclei was determined. The results indicate that human sperm chromatin becomes cross-linked under conditions of oxidative stress and exhibits increased DNA strand breakage, yet the rate of pronucleus formation is no different from that of untreated control cells. The ability of genetically damaged spermatozoa to achieve normal fertilization following ICSI has implications for the practice of this form of assisted conception therapy.