Diffuse fibrosing alveolitis is a chronic, progressive respiratory disease of undetermined cause and possibly of multiple etiologies. A proportion of affected cattle are seropositive for precipitating antibodies to Micropolyspora faeni, and this condition may represent the end stage of hypersensitivity pneumonitis. Other than the respiratory signs, the animals appear alert and maintain a good appetite until the onset of heart failure in the terminal stages. Signs include coughing, increased respiratory rate, dyspnea, and weight loss. Necropsy findings include right ventricular hypertrophy, interalveolar fibrosis, obliteration of the alveolar spaces, alveolar hyperplasia, bronchitis, and bronchiolitis. There is no treatment.