Primary influenza pneumonia is a disease of high mortality. Differentiation between a bacterial and viral etiology for the pulmonary infiltrates frequently presents a diagnostic dilemma. Rapid diagnosis is essential; and once established, the patient requires careful supportive therapy. This report describes two patients with primary influenza pneumonia complicated by severe respiratory failure. Early diagnosis of the pulmonary infiltrates was accomplished by obtaining bronchial secretions with fiberoptic bronchoscopy and staining them with influenza-A fluorescent conjugate. One patient died of the complications of this illness, despite maximal supportive therapy. The other patient survived the illness and at the time of hospital discharge was showing marked improvement in his exercise tolerance. There was subsequent gradual return of ventilation and gas transport function to normal values.