Genetic factors involved in susceptibility to insect bite hypersensitivity (IBH) in Belgian Warmblood horses (BWP) were investigated. Data relating to 3409 horses were collected using a questionnaire, administered to owners during sport competitions, BWP breeding days, breeder visits and after phone calls. Horses were classified as IBH-affected or unaffected, based on two 'disease classifiers': a lifetime record, based on owner information (life_status) and another based on whether or not the horse was showing clinical signs at the time of questioning (clin_status). IBH prevalence was 10% based on life_status, and 6.2% based on clin_status. The heritabilities estimated using threshold animal models varied from 0.65 to 0.78 on the underlying scale (0.18-0.26 on the observed scale). These research findings indicate that susceptibility to IBH is a heritable trait in BWP.