In a survey of 454 families with patients affected with Duchenne muscular dystrophy (DMD) we have found 4 genealogies with 2 or more affected patients who were related through paternal lines. In 1 of these families, 2 affected cousins showed different DNA deletions suggesting 2 independent mutations; in the other 2, in which only the propositus could be studied DNA deletions were also detected in the dystrophin gene. In the last one, with 3 affected patients, no DNA deletions were detected but immunohistochemical study of muscle biopsies showed a negative dystrophin pattern typical of DMD. Although one of these families might have occurred by chance, the probability of finding the other 3 in our sample of families with DMD patients constitutes a rare event. It is suggested that other mechanisms, such as the presence of transposable elements in other sites of the genome, could be responsible in some families, for a greater predisposition for the occurrence of pathogenic deletions, duplications or mutations.