duchenne muscular dystrophy (dmd) and becker muscular dystrophy (bmd) are caused by mutations in the dystrophin gene. we studied 106 patients with a diagnosis of probable dmd/bmd by analyzing 20 exons of the dystrophin gene in their blood and, in some of the cases, by immunohistochemical assays for dystrophin in muscle biopsies. in 71.7% of the patients, deletions were found in at least one of the exons; 68% of these deletions were in the hot-spot 3' region. deletions were found in 81.5% of the dmd cases and in all the bmd cases. the cases without deletions, which included the only woman in the study with dmd, had dystrophin deficiency. the symptomatic female carriers had no deletions but had abnormal dystrophin distribution in the sarcolemma (discontinuous immunostains). the following diagnoses were made for the remaining cases without deletions with the aid of a muscle biopsy: spinal muscular atrophy, congenital myopathy; sarcoglycan deficiency and unclassified limb-girdle muscular dystrophy. dystrophin analysis by immunohistochemistry continues to be the most specific method for diagnosis of dmd/bmd and should be used when no exon deletions are found in the dystrophin gene in the blood.