Two cases of sudden death of young people in apparently good health are reported. The only pathologic change found was a fibromuscular dysplasia of the artery supplying the conduction system of the heart with an important narrowing of the lumen and strong thickening of the arterial wall. The first case was of a 12-year-old girl who died suddenly while skiing; the second was of a 32-year-old man who died while talking to his wife. No other pathologic changes were found at autopsy, and the results of toxicologic analysis were negative. There was no individual or family history of cardiac diseases. These cases illustrate the importance of an analysis of the conduction system, including examination of the intramural coronary arteries supplying the conduction system.