In birds, as in many other taxa, the level of genetic similarity between parents is an important source of variation in offspring fitness. The majority of avian studies which explore the influence of mates’ genetic similarity on offspring viability have focused on hatching success as a viability measure. Yet, viability benefits may extend into later life stages, including the nestling period and beyond. Here, we analysed data from free-living blue tits (Cyanistes caeruleus) across three breeding seasons. We investigate whether parental genetic similarity influences offspring viability in terms of body mass, tarsus length and immunocompetence. Additionally, using brood size manipulation we tested whether environmental conditions influence the magnitude and direction of the relationship between parental genetic similarity and offspring performance. Sixteen microsatellite markers were used to measure genetic similarity between biological parents. We found that a higher genetic similarity of parents negatively affects offspring immune response to a novel antigen - phytohaemagglutinin, and this effect was independent from the experimental brood size manipulation.