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We studied nutritional ecology of American black bears (Ursus americanus Pallas, 1780) in Maine, including active and hibernating bears during 5 years, across three study areas, using nitrogen stable isotope analyses of blood samples (n = 152). Our central finding, in two study areas, is positive correlation between body mass and δ15N. This suggests use of large body size to acquire or guard food resources that have relatively high δ15N, consistent with importance of ungulates as food for the largest bears in Maine. In these two study areas, hibernating bears across the spectrum of body mass showed greater δ15N during 2 years of beechnut (Fagus grandifolia Ehrh.) scarcity compared with 2 years of beechnut abundance. Adiposity, measured by serum leptin, was greater in hibernating bears following a season of beechnut abundance compared with one of beechnut scarcity. Total litter mass correlated positively with maternal serum leptin and negatively with maternal δ15N, supporting the importance of mast, including beechnuts, to reproductive success of bears in Maine. In the third study area, bears across the spectrum of body mass had greater δ15N in all years, consistent with food resources relatively high in 15N that were available to bears of all sizes.