Potential use of Xe gas as a contrast agent for computed tomography was studied. Short- and long-term side effects during and after inhalation of premixed Xe/O2 (28-47%) from 12 studies were reported. While all but 1 subject tolerated Xe inhalation without ill effects, that individual did experience unpleasantly severe dysesthesias and a brief period of unresponsiveness. Further human studies with Xe inhalation should be conducted to explore possible early indicators for reduced tolerance of Xe by certain individuals.