Narrowing of the left main coronary trunk, which was compressed by the dilated pulmonary artery, was associated with atrial septal defect in three adults. One of them had severe pulmonary hypertension. Coronary angiograms revealed localized narrowing of the left main coronary trunk, and the left main coronary trunk had a concave shape. No stenosis of other coronary arteries was observed. In all patients the atrial septal defect was closed with a polytetrafluoroethylene patch. In the patient with 75% narrowing of the left main coronary trunk, aorta-coronary bypass was performed; it was not performed in the two with 50% narrowing. In two survivors postoperative coronary angiograms showed that the narrowing of the left main coronary trunk improved or disappeared. These results suggest that markedly dilated pulmonary arteries easily compress the left main coronary trunk and cause narrowing, which improves after atrial septal defect closure.