REASONS FOR PERFORMING STUDY:The assessment of belief that equine conformation is associated with performance and durability is a fundamental concept of horsemanship. Surprisingly, there is almost no quantitative evidence to support these beliefs. OBJECTIVES:To assess the prevalence and heritability of conformational traits in Thoroughbred yearlings, and investigate their significance for subsequent turf flat-racing performance and durability. METHODS:Nine selected conformational traits were assessed in a consistent, qualitative manner by a single veterinary observer and entered into a database together with details of pedigree and racing records. RESULTS:Conformational data were collected from 3916 Thoroughbred yearlings sold at public auction during the 7 year period 1993-1999. Most of the horses (72%) raced in the UK in turf flat races; just 7% of the yearlings failed to race. Prevalence of conformational defects for the UK horses was reported, with turned out feet the most commonly recorded defect (30% of all horses). There was a tendency towards a greater proportion of horses with defects in the group of unraced horses compared with horses that raced, but this was not statistically significant. There were some significant associations between racing performance and conformational defects but these were found to be almost completely explained by an effect of sire. All of the conformational traits showed considerable evidence of genetic influence, with heritability indices ranging 0.16-1.00. CONCLUSIONS AND POTENTIAL RELEVANCE:Overall, there were only weak associations between performance and conformation that could not be accounted for by the very strong relationship between pedigree and conformation. Further study of potential association between highly heritable conformation traits and racing durability and racing performance should be undertaken utilising validated, quantitative methods and technology.