We studied dystrophin in three young girls with a sporadic myopathy of early onset, manifested by mild to severe limb weakness, calf hypertrophy, high serum creatine kinase, normal karyotype, and morphologic features in muscle consistent with muscular dystrophy. DNA analysis did not reveal a deletion of the dystrophin gene. Immunohistochemical studies of dystrophin in muscle biopsies showed a mosaic of fibers with and without dystrophin, and immunoblot analysis showed partial dystrophin deficiency in all three patients, more severe in the patient with the highest proportion of dystrophin-deficient fibers. These observations suggest that the patients are Duchenne muscular dystrophy carriers. The data also support the concept that uneven lyonization in muscle is responsible for the clinical myopathy in these patients. We suggest that any girl with sporadic proximal limb weakness should be evaluated as a possible Duchenne carrier by dystrophin studies.