Study of 55 cases of progressive muscular dystrophies (34 Duchenne, 12 Duchenne with residual dystrophin and 9 Becker patients) comparing age, age at the initial symptoms, duration of symptoms, levels of serum enzymes, degree of disability measured by the Vignos and Archibald scale, and the type and amount of dystrophin found in the muscle biopsies by immunofluorescence. Statistical analysis showed a tendency of the symptoms and progression of disease to be related with the low quantity of dystrophin in the biopsies. There was no difference in the parameter analysed between the Duchenne patients with, and without residual dystrophin, as well as the Duchenne with residual dystrophin and Becker patients. There was an inverse relation with the amount of dystrophin and¹ the endomysial connective tissue and fatty infilration, and a direct relation with hypertrophic fibers and atrophic angulated fibers in the NADH-tetrazolium reductase. In the comments a discussion is made about the difficulties in differentiate Duchenne and Becker dystrophies, the cases with residual dystrophin and the importance of the correct diagnosis.