Glucose-6-phosphate dehydrogenase Mediterranean (G6PD Med) is a common G6PD variant around the Mediterranean Sea characterized by severe enzyme deficiency and B-like electrophoretic mobility. The molecular basis of G6PD Med is a single C-->T transition at nucleotide (NT) 563. A polymorphic site exists in exon 11 of G6PD gene: in the wild-type NT 1311 is a C (1311C), but in some individuals from diverse populations a T (1311T) is present instead. Recent studies suggest that this C-->T transition is in linkage disequilibrium with G6PD Med genotype. In the normal population of Southern Italy (Sicily and Calabria) who is at risk for G6PD Med the NT 1311 allelic frequency is 16.7-20%; no data are available for the other regions. We screened the population of Marche region, Central Italy, in order to know the percentage of this polymorphism in an area not at risk for G6PD Med: we found that the 1311T frequency is about one half of that found in Southern Italy.