The pathology of the acute and chronic stages of farmer's lung.

The pathology of five patients who had a biopsy in the acute stage is described; interstitial pneumonia, sarcoid-like granulomata, bronchiolitis, and vasculitis were found. Three of these patients progressed to the chronic stage, when the one with the most extensive bronchiolar involvement had lung function findings of airway obstruction. One progressed to the chronic stage with lung function findings of a transfer defect, and another had radiographic evidence of pulmonary fibrosis with normal lung function tests at rest. The pathology of the chronic stage is described in six patients, with necropsy findings in five. Interstitial pulmonary fibrosis, cystic change, and pulmonary hypertensive changes were the principal findings.