Haematological, clinical and some molecular genetic features of homozygous sickle cell (SS) disease in Saudi Arabia have been compared in 33 patients from the Eastern Province (Eastern) and 30 from the South Western Province (Western). Eastern patients all had the Asian beta globin haplotype whereas Western patients were more variable but predominantly of the Benin haplotype. Eastern patients had more deletional alpha thalassaemia, higher total haemoglobin and fetal haemoglobin levels, and lower HbA2, mean cell volume, reticulocytes, and platelet counts. Clinically, Eastern patients had a greater persistence of splenomegaly, a more normal body build and greater subscapular skin fold thickness, and Western patients had more dactylitis and acute chest syndrome. Painful crises and avascular necrosis of the femoral head were common and occurred equally in both groups. The disease in the Eastern province has many mild features consistent with the higher HbF levels and more frequent alpha thalassaemia but bone pathology (painful crises, avascular necrosis of the femoral head, osteomyelitis) remains common. The disease in the West is more severe consistent with the Benin haplotype suggesting an African origin.