In sexually dimorphic mammals, high population density is commonly associated with increased mortality of males relative to females and with female-biased adult sex ratios. This paper investigates the consequences of these changes on the distribution of male breeding success, the intensity of competition for females and the opportunity for sexual selection. After the red deer (Cervus elaphus L.) population of the North Block of Rum (Inner Hebrides) was released from culling, female numbers rose and male numbers declined, leading to an adult sex ratio of around one male to two females. This change was the result of increased mortality of males relative to females during the first two years of life; of increased emigration rates by young males; and of reduced immigration by males from outside the study area. The increasing bias in the adult sex ratio affected the timing of breeding as well as the distribution of mating success in males. As the adult sex ratio became increasingly biased towards females, the degree of skew in mating success (calculated across all harem-holders) increased, but mature males defended harems for shorter periods and a higher proportion of males held harems. In addition, a higher proportion of calves were fathered by immigrant males and the proportion fathered by males born in the study area declined. These results support the contention that, where high population density is associated with a female-biased adult sex ratio, competition for mates is likely to decline.