Athletes who play sports in cold weather, particularly skaters and cross-country skiers, have an increased prevalence of lower airway disease that is hypothesized to result from repeated penetration of incompletely conditioned air into the lung periphery. In this study, we investigated the hypothesis that canine winter athletes also suffer from increased prevalence of lung disease secondary to hyperpnea with cold air. Bronchoscopy and bronchoalveolar lavage was conducted in elite racing sled dogs 24 to 48 hours after completion of a 1,100-mile endurance race. Bronchoscopic abnormalities were classified as none, mild, moderate, or severe, based on the quantity and distribution of intralumenal debris. Eighty-one percent of the dogs (48 of 59) examined had abnormal accumulations of intralumenal debris, with 46% (27 of 59) classified as moderate or severe, indicating significant accumulation of exudate. Bronchoalveolar lavage obtained from dogs after the race had significantly higher nucleated macrophage and eosinophil counts compared with sedentary control dogs. Our findings support the hypothesis that strenuous exercise in cold environments can lead to lower airway disease and suggest that racing sled dogs may be a useful naturally occurring animal model of the analogous human disease.