Total creatine kinase measurement in serum has remained the best overall marker for detection and monitoring of skeletal muscle diseases, despite that different human tissues exhibit varying distributions of cytoplasmic and mitochondrial isoenzymes of creatine kinase. Acute myocardial infarction aside, increases in total serum creatine kinase, as reflected by the MM isoenzyme, are most commonly caused by injury or diseases to striated muscle. Enzyme markers of skeletal muscle injury that have been previously used (eg, aldolase, enolase, aspartate aminotransferase, and lactate dehydrogenase isoenzyme 5) are not as specific as creatine kinase and have limited clinical utility. However, new enzyme and protein markers are currently being investigated, eg, troponin and carbonic anhydrase III, which are more specific than creatine kinase toward particular tissues. Moreover, measurement of creatine kinase isoforms may provide information about whether muscle turnover is acute or chronic.