Antibodies directed against the amino- and carboxy-terminal regions of dystrophin have been used to characterize 25 Duchenne muscular dystrophy (DMD), two intermediate, and two Becker muscular dystrophy (BMD) patients. Western blot analysis revealed an altered-size (truncated) immunoreactive dystrophin band in 11 of the 25 DMD patients, in one of the two intermediate patients, and in both BMD patients, when immunostained with antiserum raised against the amino terminus of dystrophin. None of the DMD or intermediate patients demonstrated an immunoreactive dystrophin band when immunostained with an antiserum specific for the carboxy terminus of the protein. In contrast, dystrophin was detected in both BMD patients by the antiserum specific for the carboxy terminus. Quantitative studies indicated that the relative abundance of dystrophin in patients with a severe (DMD), intermediate, or mild (BMD) phenotype may overlap, therefore suggesting that differential diagnosis of disease severity based entirely on dystrophin quantitation may be unsatisfactory. Our results suggest that a differential diagnosis between DMD and BMD would benefit from examination of both the N terminus and C terminus of the protein, in addition to measurements of the relative abundance of the protein.