Eighteen of 62 patients with acute carbon monoxide poisoning showed abnormal chest roentgenograms. The ground-glass appearance was the most common finding, representing the initial manifestation. Perihilar haze and peribronchial and perivascular cuffing were the second most frequent pulmonary changes. Intra-alveolar edema was the third manifestation. Cardiac enlargement and elevation of the night hemidiaphragm were also noted. The ground-glass appearance in acute carbon monoxide poisoning might be considered as parenchymal interstitial edema caused by tissue hypoxia and/or the toxic effect of carbon monoxide on alveolar membranes. Perihilar haze was often accompanied by cardiac enlargement and seemed to be of cardiac origin. Elevation of the right hemidiaphragm indicated enlargement of the liver. The outcome following hyperbaric oxygen therapy was favorable in cases with normal chest findings or the ground-glass appearance. Perihilar haze or intra-alveolar edema signified a poor prognosis. When abnormal chest roentgen findings were noted, intensive treatment, such as 100 per cent oxygen inhalation on hyperbaric oxygen therapy, was indicated.