Gender specific behaviour resulting in enhanced reproductive success can come at an immunological cost, with males in many species exhibiting lower immune responses than female conspecifics. We aimed to investigate the immunological difference between male and female eastern grey kangaroos and to measure the effect these differences have on overall host fitness and long term reproductive success. Immunological parameters include total and differential leukocyte counts, red blood cell counts, gamma globulins, haemoglobin, packed cell volume, total serum protein and albumin concentrations. Ectoparasite and endoparasite burdens were also quantified. Reproductive success was measured by number of offspring over a given time frame and testosterone concentration. Male kangaroos showed significantly higher (p = 0.01) total leukocyte counts, as well as higher absolute lymphocyte counts (p = 0.004). Female kangaroos had significantly greater (p = 0.006) ectoparasitic burdens. Results from analysis of reproductive success will be discussed. Immunological differences are the outcome of a complex interaction of factors including the immunosuppressive effects of pregnancy and lactation, testosterone in males, and different contact rates to pathogens. Understanding sexual dimorphism of the immune system enhances our knowledge of evolutionary processes driving the development of varied traits for enhanced reproductive success and survival. This not only contributes knowledge to the field of macropod research but has flow on benefits in managing the overabundance of kangaroos. With increased knowledge of kangaroos, effective evidence based population control strategies can be implemented to alleviate the deleterious effects of overabundance in both urban and rural settings.