To define the natural history of sickle cell anaemia a cord blood screening programme was initiated and during the first 3 years a total of 129 newborns with FS phenotype were detected, of whom 83 infants were followed up regularly. Forty-three of them had attained the ages of between 1 and 3 years and the analysis of their data revealed that in 21 of 43 cohort children (14 FS and 7 FS + Barts) who had almost identically raised levels of haemoglobin F, the clinical manifestations of the disease appeared to be influenced by the size of the RBC, with the microcytic group having a relatively milder clinical course as compared with the normocytics. Similar observations were made in about 50% (47/93) of non-cohort older children and adults with sickle-cell anaemia, suggesting that alpha thalassaemia in combination with increased levels of fetal haemoglobin was probably an important predictor of the clinical severity in patients with sickle-cell anaemia in the Eastern Province of Saudi Arabia.