Erythrocyte deformability was studied in patients with sickle cell disease, using a filtration method under standard conditions. The erythrocyte deformability index (Vrbc) from homozygous (HbSS) patients was zero compared to 0.55 +/- 0.13 ml min-1 for controls with haemoglobin genotype HbAA. In the heterozygous (HbAS) sickle cell trait the mean Vrbc was 0.21 +/- 0.16 min-1 was significantly less (p less than 0.001) than in the HbAA cases. One patient with haemoglobin genotype HbAC had a normal Vrbc index of 0.53 ml min-1 while one HbSC patient had zero Vrbc index. It appears that erythrocyte deformability is independent of both the haemotocrit and haemoglobin concentration provided the haemoglobin remains normal. The results have been interpreted in the light of variation in red-cell morphology and clinical status of the patients.