A haplotype of restriction fragment length polymorphisms in the beta S globin gene cluster was determined in six different African and Asian ethnic groups with sickle cell disease. The geographical distribution of a particular haplotype, designated Asian beta S haplotype, coincides with the occurrence of raised Hb F levels in sickle cell disease. Detailed hematologic and genetic studies of a large geographically isolated group of Orissan Indian patients with sickle cell disease and a mixed Asian Indian group with homozygous beta thalassemia provided evidence that the determinant responsible for raised Hb F levels is linked to the beta globin gene cluster. The possible nature of this determinant is discussed.