We report a case of sudden unexpected death in a 17-year-old male student showing similar clinical background and pathological findings to Reye´s syndrome. He was found following cardio-pulmonary arrest in his bed, and was immediately transferred to a hospital. However, resuscitation was not successful. He had a history of high fever of 38.3 degrees C, general malaise, myalgia, and gastrointestinal discomfort for the 2 days prior to his death, and an injection of pylazolon and medication comprising anti-emetics had been administered the day before he died. His biochemical findings showed almost normal levels of transaminase, electrolytes and protein fractions at the emergency room, but blood from the heart at autopsy revealed a high titer of the influenza A virus. Macroscopically, in addition to considerable fatty metamorphosis of the liver, concentric hypertrophy of the left ventricle, muscular bridge of left anterior descending artery, moderate coronary atherosclerosis, and mild downward displacement of the septal leaflet of the tricuspid valve were noted in the heart. Although panlobular microvesicular fatty infiltration of the liver was seen, deposition of lipid droplets was detected only in hepatocytes by frozen section of several organs. Serial sectioning of the epicardial coronary arteries showed about 50% stenosis at the distal site of the left circumflex artery, and diffuse interstitial fibrosis was evident in the bilateral ventricle and this was relatively severe for his age. In addition, the atrioventricular (AV) node artery showed severe narrowing just before entering the AV node, and downward displacement of the AV node with longitudinal elongation was also remarkable. We consider that the cause of death was sudden cardiac death rather than Reye´s syndrome (RS), and that an arrhythmogenic event due to some preceding unusual cardiac lesions may have become overt due to the influenza infection and/or some related disorders. The present case would seem to suggest that a postmortem diagnosis of RS should be determined very carefully in cases of sudden death, even if the general circumstances would seem to be consistent with RS.