Sickle cell haemoglobin (Hb S) occurs at a high frequency in the Eastern (EP), South-Western (SWP) and North-Western (NWP) Provinces of Saudi Arabia and the presentation of the Hb S is believed to exhibit clinical diversity in the different regions. Three areas of Saudi Arabia were screened to determine the frequency of Hb S and α- and β-thalassaemias and glucose-6-phosphate de-hydrogenase deficiency genes. Furthermore, in an attempt to investigate and compare the clinical and haematological presentation of sickle cell disease (SCD) in the different regions of Saudi Arabia, the individuals identified as Hb S homozygotes were investigated further. The patients were further classified on the basis of whether there was associated α- or β-thalassaemia. A severity index (SI) was calculated for each patient and the clinical presentations and laboratory findings were compared. The results showed significantly variable severity of SCD in patients from different regions. The patients from the EP generally had a mild clinical presentation, while in the SWP and NWP majority of the patients suffered from a severe disease as judged by the SI. No correlation could be established between Hb F level and SI, though the WBC level correlated positively with the SI. The lowest SI values were encountered in patients with associated α-thalassaemia who also had the lowest WBC count and MCV and the highest RBC count and packed cell volume.