The long-term effects on the egg size of breeding females that had suffered parasitism when they were nestlings are described for a wild population of Pied Flycatchers naturally parasitized by the blowfly Protocalliphom azurea larvae, a common nest-dwelling, blood-sucking ectoparasite of cavity-nesting birds in the Mediterranean region. As adults, females reared in blowfly-infested nests laid smaller eggs than their counterparts raised in nests not infested by blowflies. This relationship held irrespective of female size, condition and maternal egg size, and was random with respect to female quality and consistent across a female's lifetime. Except for egg size, no long-term effects on host longevity or other fitness components, such as lifetime reproductive success, were detected. Although the mechanisms causing long-term depression of host egg size remain unknown, the recent discovery that Protocalliphora blowflies transmit viruses to nestlings offers new avenues of research on this issue.