The differential diagnosis between autosomal recessive limb-girdle (LGMD) and X-linked Becker muscular dystrophy (BMD) is very important for genetic counseling. It has been hypothesized that all BMD patients would have dystrophin alterations and dystrophin analysis could identify the Xp21 MD. Qualitatively abnormal dystrophin is easily detectable, but it is generally associated with in-frame DNA deletions or duplications. In patients with no detectable DNA deletions, in which X-linked inheritance cannot be proved, dystrophin quantification is still the only available test for differential diagnosis. In order to assess the accuracy of dystrophin quantification test in delineating Becker patients, we analyzed dystrophin abundance in BMD patients with a positive history of X-linked inheritance and no DNA detectable mutation, as compared to patients from families with LGMD. We observed that patients from 2 among the 5 BMD families have nearly normal dystrophin, while alteration in dystrophin content was observed in patients from 2 among the 7 LGMD families studied (probably as a secondary effect of alteration in the whole dystrophin-glycoproteins complex). These results suggest that dystrophin quantification, as an isolated test is not helpful for differential diagnosis between BMD and LGMD.