Creatine kinase is the serum enzyme that shows the greatest frequency of abnormality in skeletal muscle disease. Changes in this enzyme in sex-linked dystrophy (Duchenne's and Becker's) patients and carriers; in patients with other dystrophies; and in inflammatory, toxic, endocrine, and traumatic myopathy are reviewed. The changing role of serum enzyme measurements as a consequence of newer methods of genetic diagnosis is also considered.