Recent studies have proposed selective breeding policies for preventing or controlling hip dysplasia, based upon the relationships between the hip scores of the offspring and their parents. However, these studies have ignored the possible effects on an animal's hip score of its age when it was examined and the month in which it was born. This paper describes the results of statistical analyses of large data sets of Kennel Club-registered labrador retrievers and Gordon setters. Regression modelling showed that an animal's hip score depended significantly upon the hip score of its sire and dam, and upon its month of birth. Enlarging the model by including its age when examined made it possible to quantify the significant genetic parental effects and the effects of the animal's month of birth and its age when examined.