Pericardial abnormalities remain the most common manifestation of radiation-induced cardiac disease, but coronary artery lesions are not rare. In this report we describe a left coronary ostial stenosis which appeared five years after mediastinal irradiation for breast carcinoma in a 50-year-old woman. The patient underwent coronary angiography. A pressure drop was observed as the left catheter tip engaged the left coronary ostium; so, only nonselective coronary opacifications were performed showing an isolated, marked narrowing of the left coronary ostia. During surgery, a circumferential aortotomy allowed the examination of the left coronary ostium which appeared severely stenosed. The coronary tree was otherwise normal. A termino-terminal saphenous vein graft was anastomosed on the left stem and its proximal part was implanted on the ascending aorta. The coronary ostium and the proximal part of the left main stem were excised and the macroscopic examination of the proximal part of the left coronary artery confirmed the diagnosis of severe ostial stenosis. Microscopic examination of the coronary ostium showed a severe intimal thickening without any evident lesion of the media. This intimal thickening consisted of fibrous tissue without extracellular lipid deposit. Microscopic examination of the aorta near the coronary ostium also demonstrated an intimal thickening without any lesion of the media. Coronary ostial stenosis appears to be a rare lesion; its incidence has varied between 0.13 and 2.7% in angiographic studies and there is co-existing disease in multiple coronary vessels in the majority of cases.(ABSTRACT TRUNCATED AT 250 WORDS)