The clinical and haematological features of homozygous sickle cell (SS) disease were compared in 30 Greek and 310 Jamacian patients. Deletional alpha-thalassaemia, which modifies SS disease, is rare among Greek patients, so only Jamacian patients with four alpha-globin genes were included in the control group. Greek patients had higher total haemoglobin concentration and red cell counts, and lower mean cell haemoglobin concentration (MCHC) and reticulocyte counts. They also had a more normal body build and more adults had persistent splenomegaly. Fewer had a history of leg ulceration or priapism but more reported acute chest syndrome. The comparatively mild disease in Greek patients is consistent with less haemolysis and sickling and therefore less bone marrow expansion. In the absence of amelioriating factors such as high HbF concentration or alpha-thalassaemia, these findings may be explained by the low MCHC.