Creatine kinase (CK), lactic dehydrogenase (LDH), and more recently their isoenzyme determinations (CK-MB and LDH1) have been useful adjuncts in verification of myocardial injury. To determine whether DC cardioversion affects these serum enzyme levels, we recorded total CK, total LDH, CK-MB, and LDH1 levels serially during 24 hours following elective DC cardioversion in 18 patients without cardiac ischemia. New postcardioversion elevations in total CK and total LDH levels were small and occasional: CK (one of 18 patients), LDH (four of 18 patients). Elevations of CK-MB or LDH1 following cardioversion did not develop in any of the patients. Therefore, new CK-MB or LDH1 elevations associated with arrhythmias must result from myocardial damage to DC cardioversion.