This report presents the light microscopic morphology found at autopsy in 59 patients who participated in an organized controlled trial of extracorporeal oxygenation as therapy for acute respiratory failure. Observations were recorded as objectively as possible and were analyzed by computer. The experimental therapy produced no specific alteration in the observed pulmonary lesions. Many of the lesions tabulated had significant correlation coefficients with time, all of which were higher when correlated with the duration of respiratory failure than with the duration of the entire acute illness. The rapid progression of the lesions to fibrosis is emphasized as is the predilection of both early and late lesions to involve alveolar ducts to a far greater degree than the distal alveolar spaces. A unifying mechanistic hypothesis consistent with these observations, as well as others, is that the lesions may result as much from oxygen damage as from the original acute illness.