A 10-month-old cat was diagnosed with congenital subvalvular aortic stenosis. To resolve its hypoxia, oxygen therapy was administered a couple of times a week during two months. The oxygen partial pressure in the chamber was maintained between 30 and 35%, and the time for one procedure was 12-24 hr. The animal died due to severe respiratory failure. At necropsy, the lungs were voluminous and had a rubbery texture. Histologically, large type II pneumocytes with occasional atypia had diffusely proliferated within the lungs. Interstitial fibrosis was not observed, although some alveolar septa were thickened along with fibrinous exudates and neutrophilic infiltration. The histology of these lesions was consistent with diffuse alveolar damage (DAD), which might have been partially due to oxygen toxicity.