Parasites extract part or all their resources from their host depriving them of energy that could be normally used for growth, self-maintenance or reproduction. Thus, parasites are playing a major role in the evolution of life-history traits of their host through direct or indirect fitness costs. The current experiment investigated the effect of parasitic warble flies (Hypoderma tarandi), on the life-history traits of reindeer (Rangifer tarandus tarandus). In autumn-winter 2005, 52 free-ranging female reindeer were administrated with an anti-parasite drug (treatment group), whereas 56 females remained untreated (control group). Subsequently, body mass, reproductive success and calf body mass were recorded in summer and winter the following year for all individuals. Reproductive success, measured as the probability of producing an offspring, was not affected by the treatment. However, the manipulation positively affected female body mass in the summer but not in the winter and a positive trend was observed for the calves during the same season. Overall, our findings indicate that warble flies have a negative impact on reindeer through their effect on body mass and consequently, are likely to affect reindeer life history and population dynamics.