Heart type creatine kinase isoenzyme (CK MB) was detected in the serum in 23 out of 53 patients (43%) with acute cerebrovascular, traumatic, or infectious brain damage. Electrocardiogram disclosed abnormalities suggestive of acute myocardial injury in 15 of these 23 patients. Eleven of them also showed increased LD1 activity. Subendocardial haemorrhage was detected in 3 out of 8 necropsied patients with serum CK MB activity. Among those 30 patients in whom no CK MB activity was found electrocardiographic abnormalities suggestive of acute myocardial injury were observed in 2 and increased LD1 was seen in 4 cases. The mortality was higher if either CK MB isoenzyme or electrocardiographic abnormalities suggestive of acute myocardial injury were present, compared with the patients lacking these signs (P less than 0.01). Present findings suggest that acute brain damage may secondarily cause myocardial damage more often than has been believed before. Results also indicate that a combination of acute brain damage and acute myocardial injury often indicated a poor prognosis.