Objective: The clinical records of 60 patients with the acute Stanford type B aortic dissection between January 1992 and December 2001 were retrospectively reviewed to assess the clinical significance of impaired lung oxygenation in acute aortic dissection. Methods: All patients were admitted to Yamagata Prefectural Central Hospital within 24 hours from onset and received medical treatment. Respiratory index (RI) was used as a parameter of oxygenation. The relationships between oxygenation impairment and the following clinical factors were evaluated: DeBakey type 3a/3b; false lumen patency; pleural effusions; compression atelectasis; ischemic complications; white blood cell counts; serum C reactive protein levels. Results: DeBakey type 3a/3b (p=0.004), pleural effusions (p=0.002), compression atelectasis (p=0.001) and ischemic complications (p=0.005) significantly correlated with impaired oxygenation. Maximum white blood cell counts had a weak correlation with impaired oxygenation. Maximum Serum C reactive protein levels significantly correlated with impaired oxygenation. Discussion: This study confirms that impaired oxygenation is one of the most common clinical manifestations in acute type B aortic dissection. Pleural effusions, compression atelectasis, ischemic complications, extensive dissection and inflammatory reactions including leukocytosis may be associated with impaired lung oxygenation.