Forty-two patients underwent open-lung biopsy during the early phase of acute respiratory insufficiency. Correlation between the gross appearance of the lung at operation and the microscopic findings was good. Although only fair correlation was found between lung and tracheal cultures, the findings of two positive cultures in the lung only was of utmost importance. Biopsying multiple areas from the same operation showed identical pathology in 86 per cent of cases. The mortality rate of open-lung biopsy was zero; the morbidity rate was 4 per cent. The over-all survival rate of acute respiratory insufficiency (ARI) due to trauma was 39 per cent; that of pneumonia, 11 per cent. In 17 (33 percent) patients specific diagnoses and/or specific therapies were employed as a direct result of the biopsy or the thoracotomy. The incidence and prognostic implications of fibrosis and microthromboembolism are presented and discussed. Open-lung biopsy has been extremely safe and valuable in characterizing and managing ARI.