The serum activity of creatine kinase was determined in 52 patients with hypothyroidism (19 males and 33 females), 110 patients with hyperthyroidism (27 males and 83 females) and 187 control subjects (47 males and 140 females). The creatine kinase activity in patients with hypothyroidism was on the average significantly higher than controls (p<0.005). In hyperthyroid patients, although the serum activity of creatine kinase was significantly lower as compared with controls, however, it is within the normal range. Hypothyroidism should be put into consideration in patients presented with symptoms suggestive of cardiovascular abnormality without other evidence of myocardial damage and have raised creatine kinase activity.