Background: The prevalence of semen hyper viscosity (SHV) is estimated to be between 12-29% and can lead to male factor infertility both in vivo and in vitro. Infection and high levels of seminal leukocytes may results in the development of SHV. Oxidative stress and biochemical and genetic factors can furthermore contribute to this condition.Objective: The aims of this study were (a) to determine the prevalence of subjects with semen hyper viscosity (SHV) in a large population of male partners of subfertile couples; (b) to identify any correlation between SHV and infections or inflammation of the genital tract and (c) to assess sperm kinetic parameters of SHV.Materials and Methods: Semen samples were collected and examined after liquefaction for 20 min at 37oC from patients in Andrology Laboratory of Maryam Infertility Center, Beheshti Hospital, Isfahan, Iran. One hundred fifty-eight Semen samples of male partners of subfertile couples with increased seminal viscosity and 129 Semen samples with normal seminal viscosity as controls.Results: Sperm count (38.6 vs. 20.1) and motility (45.5 vs. 31.0) were significantly lower in patients with hyper viscosity compared to the control group. There was positive correlation between SHV and infections or inflammation of the genital tract.Conclusion: OS and high seminal ROS levels in SHV are related to SHV decreased sperm parameters such as motility and increased sperm DNA damage. Hyper viscosity can impair normal sperm movement in the female reproductive tract, and can lead to decreased sperm count. SHV is most attributed to male accessory gland infection, increased levels of leukocytes, and inflammation, as well as dysfunction of the sex glands or even the immune system.