This study was conducted to find whether or not oxygen breathing induces coronary vasoconstriction in diseased as well as in normal coronary arteries. Coronary angiograms were repeated in 14 patients with coronary artery disease during successive air and oxygen breathings and the diameters of the coronary arteries as displayed by film projection were measured and compared. Arterial oxygen tension was elevated from 91.3 +/- 16.8 to 457.2 +/- 26.5 mmHg by oxygen breathing but was not accompanied by any significant changes in mean arterial pressure. The diameters of coronary arteries without significant narrowing were reduced from 2.89 +/- 1.05 to 2.65 +/- 1.02 mm by oxygen breathing (a decrease of 8.9 +/- 7.0%) and that of coronary arteries distal to a narrowing of more than 50% was reduced from 2.24 +/- 0.53 to 2.02 +/- 0.57 mm (a decrease of 10.1 +/- 10.7%). These findings suggest that oxygen breathing and constrict a large coronary artery even if it has atherosclerotic lesions and the myocardium needs more flow.