The CAT G-844A polymorphism may influence CAT transcription by modulation of the transcriptional factor binding position, and it has been postulated that the A allele is associated with decreased CAT activity in comparison with the G allele . Thus, it is reasonable to hypothesize that individuals bearing this CAT polymorphism are more susceptible to oxidative stress as they possess lower levels of CAT protein than wild-type subjects. Given the lack of AA homozygous subjects in the investigated population we could not fully evaluate the effect of this polymorphism on oxidative stress response. However, our results show that heterozygous GA water polo players had higher levels of oxidative stress than wild-type subjects, even if significant differences were only found for dROMs levels after exercise, probably due to the small size of sampled subgroups. These results were also confirmed by the examination of antioxidant enzyme activities, since heterozygous subjects had a significantly higher GPx activity before exercise, and significantly higher GPx and SOD activities after exercise. However, differences in CAT G-844A genetic background among the recruited water polo players did not affect the variability of muscle damage markers before and after exercise.