We monitored four male and seven female red deer (Cervus elaphus) from Mar. 2001 to Mar. 2003 in Białowieża National Park (BNP), Poland, to determine their home ranges in one of the best preserved old-growth forests in lowland Europe, and with large (>15 kg) carnivores present. Mean home-range size of adult males (36.0 km²) was significantly larger than adult females (8.4 km²). Seasonal home-range sizes differed significantly among seasons for both males and females, with the largest home ranges being in autumn for males (23.0 km²), and winter for females (7.1 km²). Inter-sexual differences in home ranges appeared to be influenced by differences in reproductive strategies and physiological needs. Both sexes exhibited strong range fidelity, although home ranges in different years overlapped more for individual males (93–100%) than females (71–90%). Home ranges of red deer in BNP were substantially larger than that reported in previous studies throughout Europe, suggesting that in old-growth forests with large carnivores present (i.e., the historical situation for most of Europe), red deer need large areas to meet their seasonal and annual requirements.