Reproductive performance of mammalian females depends on a broad set of extrinsic and intrinsic factors. One of the most important attributes for a female ungulate is body condition, which depends on food availability. Forage supply, in turn, is dependent on habitat quality, weather conditions, and animal density. The main objectives of our study, conducted in Augustów, Knyszyn, and Białowieża Forests in northeastern Poland from 2006 to 2011, were to quantify reproductive characteristics of female red deer (Cervus elaphus) and determine factors affecting their fertility. Targeted populations of nonmigratory deer were part of an almost natural system, which included seminatural closed forests, a multispecies ungulate guild, and large carnivores. Red deer in northeastern Poland occurred at relatively low densities and females exhibited good reproductive potential, reflected by high fertility rate, low age at 1st reproduction, and lack of reproductive senescence. Fertility was affected by age and body condition. The positive effect of body mass on female fertility declined with age, while the opposite relationship occurred with mandible fat content—its effect increased with female age. Environmental factors and population density did not affect fertility. Early survival and recruitment of young were high (73% and 52 young per 100 females, respectively). Survival of young increased with age of mother in young and prime-aged females and decreased in older females. Summer weather conditions at the time of the study were stable; therefore, their effect on female fertility was not evident. The negative effects of density dependence, which operate mainly through female body condition in this large ungulate, were weak, resulting in adequate food resources, and high fertility rates and population productivity.