Myotonic dystrophy results from expansion of a (CTG)n repeat at the 3' untranslated region of the myotonin-protein kinase gene. We show here the genomic analysis of 322 symptomatic patients with the cDNA-25 probe detecting disease specific EcoRI restriction fragments. The expansion was found in the majority of Italian and Spanish patients (92%). The implications of these results for the detection of symptomatic patients in southern Europe are discussed.