The distribution of equine leucocyte antigens (ELA) in horses affected by equine sarcoid tumours was determined and compared with unaffected controls. ELA-haplotype W3,B1 occurred more frequently in affected riding horses of Irish, Swiss and French background. The combined data for the three breeds resulted in a χ2 value of 20.35 (P<0.0005 after correction). Simultaneously, ELA-specificity W11 was more frequently found in horses of Irish background, while W5 was found in Swiss and French horses with sarcoids. The combined data for haplotype W3,B1 and/or W5 specificity demonstrated, in the genetically closely-related Swiss and French horses, a highly significant difference in the occurrence of these haplotypes between affected horses and healthy controls (χ2= 28.69, P<0.00001 after correction). Thus, the results strongly suggest that the predisposition of horses to sarcoid is associated with or linked to the major histocompatibility complex.