The first major epidemiological study of injury incidence in the UK flat racing Thoroughbred (TB), published in 1985, found lameness to be the single largest reason for days when horses failed to train. It was considered advisable to ascertain if progress has been made in reducing the problem of musculoskeletal injuries in the intervening period. To quantify injury incidence and days lost from training by 2- and 3-year-old TBs in UK training yards during 2002 and 2003. One-hundred-and-eighty-two yearling TBs were recruited at the end of 2001 and daily training and injury records maintained over the following 2 training and racing seasons. Days were defined as lost from training when a horse failed to train at a slow canter speed or faster, and could be assigned to one of 4 categories: lameness, medical, traumatic and unknown. The incidence and number of days lost due to specific injuries and medical conditions was determined by further subdividing the lameness and medical categories. The study period provided a total of 52,601 2-year-old and 29,369 3-year-old days available for training, with 2-year-olds failing to train on a significantly greater proportion of days available than 3-year-olds. Lameness was the most important condition causing horses to miss training, with stress fractures being the most important cause of lameness. Medical conditions were a relatively minor cause of days lost from training, accounting for approximately 5% of untrained days in the 2 age groups. In UK flat racehorses there has been little change in the proportion of days lost from training due to lameness over the last 20 years. This study highlights the need for further efforts to reduce the problem of lameness in the racing TB.