The aim of this study was to evaluate the level of malondialdehyde (MDA) (one of the final products of lipid peroxidation and well-known marker of oxidative stress) in semen of infertile men with type 2 diabetes and to investigate its relationship with their glycaemic control. Forty infertile men with type 2 diabetes were evaluated. The mean ages were 36.5 ± 8.0. Men with diabetes were divided into two groups. Group A (n = 20) with glycated haemoglobin >10% and group B (n = 20) with glycated haemoglobin <7%. A single sample was examined according to the criteria of the World Health Organization (WHO Laboratory Manual for the Examination of Human Semen and Sperm-Cervical Mucus Interaction, 1999, Cambridge University Press). MDA was assessed using the thiobarbituric acid method. MDA concentration in semen of group A patients (0.95 ± 0.35 nmol ml(-1)) was significantly higher than in group B patients (0.43 ± 0.13 nmol ml(-1)) (P value < 0.05) and had negative relationship with sperm density (r = -.717; P value < 0.05), total sperm count (r = -.625; P value < 0.05), progressive motility (r = -.489; P value < 0.05) and normal forms (r = -.545; P value < 0.05). Based on these results, it could be concluded that increase in lipid peroxidation in men with diabetes with poor metabolic control was associated with low sperm quality.