Starting from a group of 265 Italian patients affected with Duchenne or Becker muscular dystrophy a screening for duplications in the dystrophin gene was performed on 112 cases in which no deletions had previously been detected. The 21 intragenic duplications detected account for 7.9% of the total. Among these, one duplication including exons from 3 to 43 is the largest reported so far. Data from this study were combined with those from the literature and breakpoint distribution by intron was analysed. In general breakpoints occur mostly in the proximal third of the gene, in particular in intron 7. However, both the frequency of duplications and the distribution of breakpoints by intron are different in the Japanese sample compared with the other groups of patients. The role of geographical differentiation of intron sequences by genetic drift and of transposon-like sequences in explaining these differences is discussed.