Analyses of deletions in the dystrophin gene and of cognitive status were performed on patients with Duchenne (DMD) or Becker (BMD) muscular dystrophy in order to find a correlation between both features. Molecular study by multiplex and simplex PCR of dystrophin exons led to the identification of 51 deletions in 126 unrelated patients. Most of them were frameshift, in full agreement with severe clinical symptoms, three patients with a BMD-like phenotype had in-frame mutations. Deletions were localized with reference to the different dystrophin isoform sequences and were clustered in two main areas, 5' and central+ 3' end of the gene. Cognitive abilities were tested in 47 out of 51 patients with identified mutations, 23 of them being mentally impaired. Comparison of molecular and neuropsychological features showed that deletions localized in central and 3' parts of the gene (18 out of 23) are preferentially associated with mental impairment. Fourteen of them were found in the regulatory and coding sequences for the three CNS specific carboxy terminal isoforms. Therefore, though mutations with variable locations may lead to cognitive impairment, our results show that deletions in the distal portion of the gene are basically related to mental retardation.