Summary. The haematological indices in 50 patients with homozygous sickle cell (SS) disease and avascular necrosis of the femoral head and in 180 patients with SS disease without this complication have been compared. Avascular necrosis of the femoral head was significantly associated with high total haemoglobin levels and with low fetal haemoglobin levels. Low fetal haemoglobin levels are associated with increased intravascular sickling. These results suggest that both intravascular sickling and the increased viscosity associated with high haemoglobin levels are related to the development of avascular necrosis of the femoral head in SS disease.