The diagnostic yield and risks of transbronchial biopsy (TBB) during mechanical ventilation were assessed in 13 patients with progressive pulmonary infiltrates. TBB was of considerable diagnostic value in ten patients and useful in excluding potentially treatable infections in the remaining three patients. Complications included two pneumothoraces, pulmonary hemorrhage in one case, and supraventricular tachycardia in another. No fatalities were attributable to TBB. TBB proved to be a relatively safe procedure, with a high diagnostic yield in these critically ill patients.