This thesis is an investigation into the possible genetic background underlying the liability of Standardbred racehorses (Std) to atrial fibrillation (AF). Pedigrees of 204 affected Std admitted to the Ontario Veterinary College Teaching Hospital (OVCTH) for treatment of AF born from 1978 to 2007 and of 1,017 randomly selected racing contemporaries (5 for each Std admitted). Estimates of heritability of AF were obtained using a linear threshold animal model. Marginal genetic contributions of ancestors to affected and control cohorts were determined and differences were tested. The estimate of heritability of AF in the Std was 14% on the observed scale. Eleven ancestors had significantly higher contributions to affected cohorts than to controls. Many of these ancestors appeared multiple times in the five-generation pedigrees of affected horses. Results strongly indicate a genetic predisposition to AF in the Std, with the arrhythmia particularly prevalent in one popular sire line.