Simultaneous use of pathologic and microangiographic methods in 47 patients with lung contusions due to a closed chest injury with no parietal lesions differentiated between: early traumatic lesions consisting in alveolocapillary ruptures and hematomas distributed at random throughout both lungs, with immediate disseminated intraalveolar hemorrhage as a result; and secondary lesions, known as "shock lung", which are mainly the result of intraalveolar hemorrhage. All these lesions explain the usual development of various forms of fibrosis, responsible clinically for refractory hypoxia.