Male factor infertility is a contributing factor in up to 50% of infertile couples. Increasing numbers of couples undergoing assisted reproductive technology (ART) treatment and reports of a possible decline in male fertility suggest that lifestyle, occupational and environmental exposures might impair semen quality. Sperm DNA contains both DNA strand breaks and base damage that has been associated with poor semen quality but few studies have examined the role of double strand breaks (DSBs), a toxic lesion, or DNA damage such as N7-methyldeoxyguanosine (N7-MedG) arising from alkylating agents that can be toxic and mutagenic. The aim of this research was to examine the relationships between exposures, DNA damage and male fertility.Men were recruited from couples attending for ART treatment and they provided information on lifestyle, occupational and environmental exposures as well as a sperm sample. Semen concentration and motility was determined by standard techniques in the neat sample and the prepared sample that underwent density gradient centrifugation for ART treatment. DSBs were measured in individual sperm cells by the neutral Comet assay and N7-MedG levels in sperm DNA by an immunoslot blot assay. Information on ART outcomes (% fertilisation, % cleavage and clinical pregnancy) was collected and associations between DNA damage, exposures, semen quality and ART outcomes were determined. Expression of individual DNA repair proteins was also examined in individual oocytes.Men in manual work had significantly lower semen volumes and higher % immotile sperm. Exposure to dry cleaning fluids and having a fever were associated with a decrease in sperm number and while non-ionizing radiation was associated with an increase in % immotile sperm, X-ray exposure was correlated to a decrease in % progressively motile sperm. Semen parameters were significantly and negatively correlated with DSBs in neat and prepared sperm, and N7-MedG levels in neat sperm. Density gradient centrifugation improved sperm sample quality and decreased DSBs and N7-MedG levels significantly. Successful fertilization of oocytes was negatively associated with DSB levels in neat and prepared sperm and with N7-MedG levels in neat sperm. Lower DSB levels in men were associated with an increased chance of an achieving clinical pregnancy especially in ICSI couples. N7-MedG levels were significantly correlated with driving a car and exposure to detergent or printing inks and dyestuffs. DSBs were correlated negatively with exercise and positively with eating nuts and almonds or exposure to non-ionizing radiation. DNA repair gene expression in individual oocytes showed significant intra and inter-individual variability.Sperm DNA damage can reduce male fertility, but the causes of such damage remain to be identified. The variable ability of individual oocytes to repair this damage may well affect the chance for a successful pregnancy.