Studies on the mutation 563T and silent mutation 1311T of the glucose-6-phosphate dehydrogenase (G6PD) gene in Poland were performed in 26 families affected with G6PD deficiency classified-according to WHO-as group 2 G6PD deficiency. Both mutations were found in 19 families, including 17 of Polish origin. Mutation 563T alone was found in 1 Greek female. The frequency of the silent mutation 1311T in Polish unaffected controls was 0.10. It is postulated that at least parts of the Polish (or Middle-Eastern European) and Mediterranean populations are of a common origin.