The associations between reproductive effort, as assessed by variation in natural clutch size and experimentally altered brood size, breeding success, and prevalence of blood parasite infections were studied in blue tits, Parus caeruleus. Females infected with blood parasites during incubation had laid significantly smaller clutches than non-infected females, irrespective of female age and laying date. Infected and non-infected females did not differ in the length of incubation delay, the time spent incubating eggs, or in fledging success. However, nestlings reared by females that had infections during incubation fledged in significantly poorer condition than nestlings reared by non-infected females. These findings are consistent with the interpretation that parasites had a negative impact bn their blue tit hosts, but they do not exclude the possibility that the prevalence of haematozoans and reduction in reproductive output (clutch size) and success (fledging condition) were independently triggered by some third factor (e.g., energetic stress caused by variation in territory quality). However, the prevalence of parasites at day 14 posthatch was related to experimentally altered brood size: adult females and males rearing enlarged broods were more likely to be infected than those rearing control or reduced broods, although the opposite was true for juvenile females and males. From our results, we suggest that a naturally low reproductive output (clutch size) may be an indicator of low individual quality, and that there may be a trade-off between reproductive effort and immunocompetence.