There are only a few recent studies that have demonstrated senescence in ungulates and nothing is known regarding how patterns of senescence may vary as a function of density Senescence in general is linked to the cost of reproduction, which probably increases with density in ungulates and may differ between the sexes. Further, senescence in ungulates is also regarded to be a function of tooth wear rates. As density dependence and sexual differences in food choice have been well documented, this may lead to different tooth wear rates and, thus, possibly density-dependent and sex-specific patterns of senescence. We therefore investigated the effects of age, sex, density and their possible interactions on the variability of body weight in 29,047 red deer harvested during 1965-1998 from Norway, out of which 380 males and 1452 females were eight years or older. There was clear evidence that spatio-temporal variation in density correlated negatively with body weights. In addition, there was evidence of senescence in both male and female red deer. Age at onset of senescence in females was after 20 years of age and independent of population density. In males, the onset and rate of senescence increased with increasing population density. The onset of senescence for males was at ca. 12 years of age at low density, but decreased to approximately ten years of age at high density. The pattern of density-dependent senescence in males, but not that in females, can be explained if (i) the cost of reproduction increases with density more strongly in male than in female red deer, and/or (ii) tooth wear rates are density dependent in males, but not in females. We discuss the ability of these two different, not mutually exclusive hypotheses in explaining the observed pattern of senescence.