There is good evidence in literature that intrauterine insemination (IUI) is the best first line treatment and most cost-effective procedure for moderate male factor subfertility. It seems very difficult to identify individual semen parameters predicting the likelihood of pregnancy after IUI. This can be explained by a lack of standardization of semen analysis, but many other methodological variables may also influence IUI success rates such as the patient selection, type of ovarian stimulation and number of inseminations per cycle. A review of the literature confirmed that sperm morphology using strict criteria and the inseminating motile sperm count (IMC) after sperm preparation are the two most important sperm parameters to assess the real impact of semen quality on IUI outcome. A universal threshold level above which IUI can be performed with acceptable pregnancy rates has not been determined yet, although IUI success seems to be impaired with <5% normal spermatozoa and an IMC of <1 x 10(6). Until now, no method of sperm preparation has been shown to be superior with regard to pregnancy rate after IUI. Whether supplementation of culture media with substances such as antioxidants and platelet activating factor may improve the results remains the subject of further research.