Background: Mode of inheritance of equine recurrent airway obstruction (RAO) is unknown. Hypothesis: Major genes are responsible for RAO. Animals: Direct offspring of 2 RAO-affected Warmblood stallions (n 5 197; n 5 163) and a representative sample of Swiss Warmbloods (n 5 401).Methods: One environmental and 4 genetic models (general, mixed inheritance, major gene, and polygene) were tested for Horse Owner Assessed Respiratory Signs Index (1-4, unaffected to severely affected) by segregation analyses of the 2 half-sib sire families, both combined and separately, using prevalences estimated in a representative sample.Results: In all data sets the mixed inheritance model was most likely to explain the pattern of inheritance. In all 3 datasets the mixed inheritance model did not differ significantly from the general model (P 5 .62, P 5 1.00, and P 5 .27) but was always better than the major gene model (P o .01) and the polygene model (P o .01). The frequency of the deleterious allele differed considerably between the 2 sire families (P 5 .23 and P 5 .06). In both sire families the displacement was large (t 5 17.52 and t 5 12.24) and the heritability extremely large (h 2 5 1). Conclusions and Clinical Relevance: Segregation analyses clearly reveal the presence of a major gene playing a role in RAO. In 1 family, the mode of inheritance was autosomal dominant, whereas in the other family it was autosomal recessive. Although the expression of RAO is influenced by exposure to hay, these findings suggest a strong, complex genetic background for RAO.