Certain clinical and necropsy findings are described in 16 young (aged 15 to 33 years) patients who received greater than 3,500 rads to the heart five to 144 months before death. All 16 had some radiation-induced damage to the heart: 15 had thickened pericardia (five of whom had evidence of cardiac tamponade); eight had increased interstitial myocardial fibrosis, particularly in the right ventricle; 12 had fibrous thickening of the mural endocardium and 13 of the valvular endocardium. Except for valvular thickening, the changes were more frequent in the right side of the heart than in the left, presumably because of higher radiation doses to the anterior surface of the heart. In six of the 16 study patients and in one of 10 control subjects, one or more major epicardial coronary arteries were narrowed from 76 to 100 percent in cross-sectional area by atherosclerotic plaque; one patient had a healed myocardial infarct at necropsy and one died suddenly. In 10 patients and in the 10 control subjects, the four major epicardial coronary arteries were examined quantitatively: 6 percent of the 469 five millimeter segments of coronary artery from the patients were narrowed from 76 to 100 percent (controls = 0.2 percent, p = 0.06) and 22 percent were narrowed from 51 to 75 percent (controls = 12 percent). The proximal portion of the arteries in the patients had significantly more narrowing than the distal portions. The arterial plaques in the patients were largely composed of fibrous tissue; the media were frequently replaced by fibrous tissue, and the adventitia were often densely thickened by fibrous tissue. In five patients, there was focal thickening (with or without luminal narrowing) of the intramural coronary arteries. Thus, radiation to the heart may produce a wide spectrum of functional and anatomic changes but particularly damage to the pericardia and the underlying epicardial coronary arteries.