The lungs of 7 horses with the syndrome called "heaves" and 6 horses used as controls were examined morphologically. The horses with heaves were found to have bronchiolitis, often with a prominent eosinophilic component, excess bronchial mucus, and perbronchial fibrosis with epithelial metaplasia. These features were not present in all animals, but it is postulated that the lesions observed were different manifestations of essentially the same condition. Heaves is considered to be an allergic phenomenon related to inhalation of some component of moldy hay, and it resembles asthma, eosinophilic pneumonitis, and farmer''s lung in the human. Widespread destructive emphysema was not a feature of the horses with heaves.