A 30-year-old man showed exercise-induced myalgia, calf muscle hypertrophy and high serum creatine kinase level, without muscular weakness. The symptoms began in childhood and did not progress. Electromyographic findings were consistent with myopathy, and the muscle histology showed nonspecific myopathic changes without evidence of storage of glycogen or lipid. Immunohistochemical staining with antibodies raised against three different dystrophin peptides revealed proper subcellular localization of dystrophin at the sarcolemma of all myofibers, but the intensity of the stain was decreased. Western blot analysis using the same antibodies revealed normal dystrophin in size, but showed the reduced amount of the protein. The DNA obtained from the patient's peripheral leukocyte showed a deletion of the dystrophin gene including exon 45 by using PCR technique. Though the detailed size and portion of the deleted gene are not ascertained, the deletion in this case may not severely affect the function of dystrophin, unlike cases of Duchenne or Becker muscular dystrophy. Dystrophin analysis is useful for the etiologic diagnosis in cases of myalgia and high CKemia.