We report an unusual case of "arrhythmic" sudden cardiac death in a 3-year-old child who died of ischemic myocardial lesions as a result of intimal fibroplasia of the coronary arteries. Also affected were the aorta and its major branches, whereas renal and mesenteric arteries, celiac trunk, and systemic veins were normal. Histopathologic examination showed severe concentric thickening of intima because of a proliferation of spindle-shaped cells (mesenchymal cells) set in an abundant extracellular matrix. In some vascular segments the intima was densely fibrotic and hyalinized. No significant inflammation, foam cells, cholesterol clefts, or other evidence of atheroma were present. The intimal lesions did not involve the media and/or the adventitia. Immunohistochemical staining of intima showed the proliferating mesenchimal cells to be myofibroblastic. Reactions for vimentin and smooth muscle actin were positive, while those for desmin, myosin, CD34, and Factor VIII were negative.