In October 2001, a survey was conducted about cavities formed within the hoof wall (called GIDOH in Japanese) of racehorses at the Ritto and Miho Training Centres, which are managed by the Japan Racing Association (JRA). Gidoh is defined as a progressive cavity within the deep layers between the stratum medium and stratum internum. A total of 148 out of 5386 surveyed horses (2.75 per cent) were affected. Out of 244 affected feet, fore hooves (84.02 per cent) were more susceptible than hind hooves, and the site most affected was midline dead centre of the toe (59.62 per cent) which tends to place extra stress at the break-over point in a straight-line exercise. Logistic regression analysis revealed that prevalence was significantly related with horse affiliation (OR 0.65, 95 per cent CI 0.46 to 0.91) and age (OR 1.43 per one year, 95 per cent CI 1.27 to 1.61). We hypothesised that the primary cause of Gidoh development in JRA stables was mechanical deformation of the hoof wall during exercise, and secondary exciting causes can subsequently lead to the spread of the lesion over the entire hoof wall.