Supplementary feeding studies are widely used to assess the effects of food availability on herbivore population dynamics. Supplementary feeding studies make the implicit and often untested assumption that supplementary feed is used by the target population. Here we describe and present the results of a supplementary feeding experiment to assess the effects of over-winter food availability on mountain hare Lepus timidus body condition, fecundity and survival in two fed and two control areas. We used passive induced transponder (PIT) tags and feeding stations equipped with PIT tag readers and data loggers to monitor individual use of supplementary feed. Fifty per cent, of 119 PIT-tagged hares, which were resident on the fed areas, used food, but individual variation in the time spent feeding was large. Food supplementation was associated with greater male body mass, earlier breeding, higher fecundity and longer survival. At the population (treatment) level these differences were not statistically significant. At the individual level the combined radio-telemetry and PIT tag data revealed a large and highly significant effect of supplementary feeding on survival. Recent syntheses of mountain hare population ecology have not identified food as a key factor determining dynamics. Our experimental study however demonstrates that food may have profound effects on individuals. In addition our study raises critical questions about the design and interpretation of supplementary feeding studies.