Winter den abandonment by brown bears Ursus arctos in south-central Sweden and southeastern Norway was found to occur in 9% of 194 bearwinters, based on 68 radio-marked bears almost two years old and older. There was no statistical difference between the sexes, between adults and subadults, nor did protection from military or timber-harvesting activities reduce the rate of abandonment. Although anecdotal, observations suggest that human disturbance was a major cause of den abandonment. Most abandonment occurred early in the denning period, before mid-winter. Bears moved up to 30 km before denning again. Distance was not related to sex, age, or time of abandonment. Apparently for the first time, a fitness cost of den abandonment is documented: pregnant females that changed dens prior to parturition lost young in or near the den significantly more often than those that did not move.