The aim of this study was to investigate the effect of cumulative mild head injury on the cognitive functioning of professional rugby players. A comprehensive battery of neuropsychological tests was administered to 26 professional rugby players and to a comparison group of 21 professional cricket players. The group test results of the rugby, cricket, rugby forwards and rugby backline players were each compared with established normative data. Generally, the comparison of the rugby and cricket mean scores relative to the normative data did not reveal significant differences on tests known to be sensitive to the effects of mild head injury. However, the comparison of variability for each of the rugby and cricket playing groups relative to variability for the normative data, revealed a pattern of increased variability among the rugby players. This implies a bimodal distribution in which a significant number of rugby players were performing poorly across these tests whereas a significant proportion were not. This variability effect was accounted for by further mean score comparisons which revealed that, as a group, it was the forward players whose performances were disproportionately poor on tests sensitive to the effects of mild head injury. The implications of these results are developed theoretically within the context of brain reserve capacity theory and suggestions for future research are provided.