The gamma-globin chains of haemoglobin F are encoded by two non-allelic genes which synthesise two different gamma-chains referred to as G gamma and A gamma. Xmn I restricts on both sides of the genes and normally produces an 8.1-kilobase (kb) fragment containing both gamma-genes. A polymorphic site on the 5' end of the G gamma genes has been reported in some populations and it results in the production of a 7.0-kb fragment containing both gamma-genes. We investigated the Xmn I polymorphic site in normal individuals, haemoglobin S heterozygotes and sickle cell disease patients from three different regions of Saudi Arabia. In the eastern province, 10% of gamma-genes in normal individuals were linked to the 7.0-kb fragment while in the sickle cell disease patients and haemoglobin S heterozygotes the frequency of the polymorphic site was 0.932 and 0.625, respectively. In the south-western province, the frequency of the polymorphic site in normal individuals was 0.096 (similar to that in the eastern province) but in the sickle cell disease patients and sickle cell heterozygotes the frequency was 0.033 and 0.095, respectively. Finally, only 6 sickle cell disease patients from the north-western province were investigated and all had the gamma-globin gene linked to the 8.1-kb fragment. The results of this study in different regions of Saudi Arabia are presented and the possible significance of Xmn I polymorphism is suggested.