Inbreeding, the mating of related individuals, has been the one dominating breeding strategy for dog breeders during the last 200 years. However, its negative effects on fer-tility and other fitness traits, a phenomenon called inbreeding depression, have been ignored in the quest of perfect unity in conformation or performance. The purpose of this study was to test the hypotheses that litter size is reduced as a consequence of in-breeding, affected by both the inbreeding coefficient of the dam and the litter and that a rapid increase in the inbreeding coefficient affects litter size more than a slower rate. Data were collected from the Swedish Kennel Club registry of origin. Breeds included in the study were the Bernese Mountain Dog, the Cocker Spaniel, the Golden Retriever, the Rottweiler and the West Highland White Terrier.