SummarySerum adenylate kinase (AK) activity was measured in cerebral vascular diseases and the AK isoenzyme pattern was studied in the patients suffering from cerebral infarction, cerebral hemorrhage, myocardial infarction, and polymyositis. There was a significant difference in AK activity between the control subjects and patients with acute cerebral infarction. Serum AK activity showed a peak on the 2nd day to the 4th day after the stroke attack, maintained at high level for about one week, and decreased gradually thereafter. There were three isoenzymes (AK-1, AK-2, AK-3) of AK, and we found that the AK isoenzyme patterns were different in different disease. The AK isoenzyme patterns in patients with cerebral infarction and cerebral hemorrhage were different from those of control subjects. The AK-3 isoenzyme was dominant in the control subjects. The proportion of AK-1 isoenzyme was increased in cerebral infarction and cerebral hemorrhage compared with that for the control subjects, and the proportion of AK-2 isoenzyme was elevated in acute myocardial infarction and polymyositis. These results suggest that serum AK is a sensitive indicator of cerebral infarction and a useful marker to follow the course of patient recovery. Moreover, the analysis of AK isoenzymes is useful in the diagnosis of cerebral vascular diseases.Key Words: adenylate kinase, isoenzyme, cerebral vascular disease There are many reports concerning elevated creatine phosphokinase (CPK) activity and CPK isoenzymes in acute cerebral vascular diseases (CVD) [1][2][3]. Previously, we studied CPK activity and CPK isoenzymes in the early stage of cerebral infarction, and found that the CPK levels remained within the normal limit in many cases and the band of CPK-BB was not elevated. On the other hand,