Although elevated serum creatine phosphokinase (CPK) activity is a sensitive indicator of myocardial necrosis, abnormal values may occur in a variety of extracardiac disorders. Fifty-two of 100 consecutive patients admitted to a short-term medical ward had elevated CPK activity; 26 of them had more than twice normal. Increased physical activity and intramuscular injections should be considered as potential etiologic factors of abnormal CPK levels in patients with chest pain, particularly when the electrocardiogram and other serum enzymes are not confirmatory. It is hazardous to diagnose myocardial necrosis on the basis of an isolated CPK elevation in the absence of other suggestive evidence. With the development of tissue-specific CPK isoenzymes, however, the clinical usefulness of this test may be enhanced.