Increased oxidative damage has been suggested to play an important role in the viscosity changes of blood. However, changes in levels of oxidative damage products in semen and their relationship to seminal fluid viscosity are unknown. The aim of our study was to investigate whether oxidative damage was associated with seminal plasma viscosity in infertile subjects. The levels of malondialdehyde, and protein carbonyls were measured in sperm and seminal plasma from 102 individuals, including 60 infertile patients. Seminal fluid viscosity and semen viscosity were studied by use of capillary viscometer and glass pipettes, respectively. Significantly higher levels of oxidative stress and damage markers were found in subfertile subjects compared with the control subjects. The seminal fluid viscosities of patients were found to be significantly higher, although all of the control and patient subjects had normal viscoelasticity when semen samples were assessed according to World Health Organization guidelines. From Pearson correlation analysis, there were significant positive correlations between seminal fluid viscosity and seminal malondialdehyde and carbonyl levels in infertile males (r =.676, P <.01; r=.276, P <.05, respectively). Our results suggest that increased oxidative damage might be a factor for hyperviscosity of seminal plasma in infertile males.