Oxidative stress is a common condition suffered by biological systems in aerobic conditions. Human semen also has its own molecular guard against the free radicals created by normal respiratory process or from immune reactions. The equilibrium of the creation and scavenging of free radicals is mandatory in the spermatozoa to fertilize and initiate a full-term pregnancy. The paper is a systematic review of publications that evaluate oxidative stress in semen. The Cochrane Library, Medline (1966-2003), Embase (1988-2003), SciSearch (1981-2003) and the conference papers were searched. When sperm samples from fertile and infertile males were analyzed, some of the mechanisms that determine the oxidative stress level were found to be impaired while others were unaltered. In conclusion, the literature as a whole provides contradictory findings and it is necessary to carry out more work to identify all the enzymatic and non-enzymatic systems involved in oxidative stress in the ejaculate, in order to develop new diagnostic systems and therapeutic strategies for combating detrimental free radical imbalance in the semen.

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