In this study, the presence and characteristics of the recurrent airway obstruction (RAO), in Chilote horses were investigated. The Chilote horse is a breed of small and very rustic horses, descendent from the horses that were brought by the spaniards during the colony and remained isolated in the Chiloe island, south of Chile, for a long time. Twenty two animals, all from the same farm and living under similar conditions, were physically examined. In all cases bronchealveolar lavage (BALF) was performed and the recovered resident cells from the respiratory tract were studied in smears stained with May Grewald - Giemsa: the level of antibodies against Aspergillus fumigatus was determined by ELISA. The results indicate that out of the total population of animals under investigation, 13 showed clinical signs compatible with RAO and 9 were normal. In all cases, the animals that were considered RAO positive showed a higer percentage of polymorphonuclear neutrophils in the BALF: furthermore, a clear positive correlation between the relative amount of neutrophilic cells present in the airways and the severity of the signs observed during the physical examination exists. All of the animals, healthy and RAO positive horses, showed high litres of antibodies against A. fumigatus, a well known allergen, commonly distributed in the region and present in the hay/straw. Our results show that Chilote horses, even though they are a rustic bread, are very susceptible to become RAO positive with similar characteristics to those described in other breeds, moreover, all animals were sensitized to A. fumigatus antigens showing the immunogenicity and the important distribution of this fungal allergen.