Research on the costs of nest reuse is central to understanding the population ecology and evolution of cavity-nesting birds. We explored the consequences of nest reuse by offering blue tits three types of nestboxes in which to breed: nestboxes with an old nest (O), empty nestboxes (E) and nestboxes with an old nest fumigated with insecticide (F). The experimental groups differed in ectoparasitism level: the O nestboxes had the most and the F nestboxes the fewest ectoparasites. Blue tits using nestboxes that contained old nests from the previous season paid a cost caused by the presence of nest ectoparasites: both reproductive success and female body mass at the end of the nestling period were reduced. Haematozoan infections in females also increased with the level of ectoparasitism. The costs of nest reuse would be reduced in areas and/or seasons with few ectoparasites, as shown by the results from the group with fumigated old nests.