Red squirrel (Sciurus vulgaris) mortality was monitored opportunistically during the period 2004 to 2013 on the island of Anglesey in North Wales. Road traffic proved a most significant cause of death (48%) mirroring the findings of earlier United Kingdom (UK) studies. Red squirrels were also found to have died from a range of pathological infections and disease previously unrecorded in Wales. These data have increased our knowledge on the national distribution of such causal factors. The study found male red squirrels were less likely to have an adenovirus infection than females and that animals dying from disease had a lower body mass than those associated with a traumatic death. No red squirrels were found with squirrelpox infection or antibodies to this virus which reinforces earlier findings from Anglesey that intensive grey squirrel (Sciurus carolinensis) culling reduced the prevalence of this infection within sympatric populations. Finally we highlight the potential intra and inter-specific infection risk presented by supplemental feeding.