Although it is well known that pulmonary parenchymal injury impairs oxygenation, the relationship between hypoxemia and volume of the contused lung after blunt chest trauma remains unknown. We studied 23 blunt chest trauma patients who underwent thoracic computed tomography (CT) within 24 hours after admission and also underwent follow-up CT at 1-week intervals until abnormal lung findings disappeared. Contused lung volume was determined quantitatively by CT. CT findings of parenchymal injury were either a diffuse lesion that disappeared within 1 week (n = 9 patients, group 1), or a solitary lesion that remained a few weeks after resolution of the diffuse lesion (n = 14 patients, group 2). PaO2/FiO2 was positively correlated with the volume of contused lung during the first 24 hours in both groups, however, there was no significant correlation after 1 week after injury. Degree of impaired oxygenation is related to the volume of the diffuse lesion that is recognized soon after injury.