AIM The aim of this study was to investigate the prevalence of diabetes mellitus (DM) in a population of male partners of infertile couples, to evaluate their seminal and biochemical parameters and to plan therapeutic strategies to achieve pregnancy. METHODS Standard semen analysis was carried out in 510 male partners of infertile couples, aged 25 to 51 years. RESULTS The prevalence of DM in the subjects studied was 1.18%, 4 cases with non insulin dependent diabetes mellitus (NIDDM) and 2 cases with insulin dependent diabetes mellitus (IDDM). Three subjects with NIDDM and 1 with IDDM had normal ejaculation. Semen analysis of these subjects showed qualitative alteration; the most important effects concerned the kinetic properties, especially progressive motility. Sperm morphology was also significantly compromised. On the other hand, sperm concentration did not show significant alterations. The therapeutic iter in these subjects is similar to that for other dyspermic male partners of infertile couples. The 2 remaining subjects (one with NIDDM and one with IDDM) had retrograde ejaculation (sperm recovery from post-orgasmic urine) and were included in intrauterine insemination or in vitro fertilization programs. In addition, in the subjects with DM, the biochemical seminal profile showed significantly higher levels of fructose compared with the nondiabetic subjects. CONCLUSION DM has a negative impact both in terms of sperm quality and of the ejaculation mechanism. The low prevalence of DM among this study population of 510 male partners of infertile couples is due to the low prevalence of DM in this age range.