A survey was carried out into white line disease in 1781 Thoroughbred racehorses kept in stables at the Japan Racing Association (JRA) Miho Training Center (MTC) September-October 1996. The survey was conducted while horses were being shod by farriers. The horses that still exhibited damaged white lines after regular trimming were diagnosed as having white line disease. The factors recorded were age, sex, number of diseased horses, number of diseased hooves, number of lesions by region over the bearing border of the hoof and the classified length of such lesions. The percentage of total diseased horses was 11.5% (204 animals), with incidence increasing significantly with age (P<0.01). Occurrence was independent of sex (P>0.05) was more frequent in the fore- than in the hindhoof and developed more frequently at the toe than at any other region of the forehoof-bearing border. Most lesions ranged from 20 to 30 mm in length.