Myoglobin, myosin creatine kinase MM (CK-MM), creatine kinase BB(CK-BB) in cardiac muscle and H chain of myosin in atrial and ventricular muscle were studied in specimens from 8 patients who died of sudden nocturnal death syndrome (SNDS) by avidin-biotin complex (ABC) method to investigate the possible early or very early myocardial ischemia in the syndrome. Hematoxylin-eosin staining was conducted for comparison. The results showed evident loss of CK-MM, CK-BB, myoglobin and myosin from cardiac muscle cells, indicating that occurrence of SNDS is closely associated with acute myocardial ischemia.