We examined the influence of body mass in early winter on litter size, growth and sex ratio of young, as well as the influence of gestation and lactation on overwinter loss of mass among female black bears ( Ursus americanus ) in La Mauricie National Park, Quebec, Canada. All adult females weighing ≥77 kg gave birth and no female reproduced when weighing <56 kg. Litter size (two to four young) was influenced by maternal condition in early winter, and overwinter loss of mass greater for females producing litters of three and four young. For a particular litter size, heavier females tended to produce more male young than expected from an equal sex ratio. Maternal condition, however, could not explain the strongly male-biased sex ratio (2.5 males: 1.0 female) at birth observed in this population. Significance of sex ratio at birth in relation to the regulation of the population of bears at La Mauricie National Park is discussed.