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The objective of this study was to evaluate the rate of acute splenic sequestration (ASSC) in patients with sickle beta-thalassaemia and sickle cell anaemia, the risk of recurrence in those who survive the first episode, and the relationship between ASSC episodes and subsequent hypersplenism. All patients with confirmed diagnosis of sickle cell disease at a tertiary referral teaching hospital, between January 1994 and December 2002 were interviewed and had their medical records reviewed. Seventy-seven patients with sickle cell disease were identified. Their ages ranged between 2 and 18 years (mean, 10.1 years). There were 35 females and 38 males. Thirty-seven (50.6%) had sickle beta-thalassaemia, and 36 (49.4%) had homozygous sickle cell anaemia. Of these, 26 had high level of Hb F and 11 had normal level of fetal haemoglobin (Hb F). Twenty-one patients (28%) had 63 episodes of acute splenic sequestration. Thirty-seven episodes were experienced by 12 patients with sickle beta-thalassaemia; of these 11 were major attacks with one fatality. Twenty-six episodes were experienced by nine patients with sickle cell anaemia. Splenomegaly and hypersplenism were greater in the acute splenic sequestration group than in the rest of the sickle cell anaemia patients, and the differences were extremely significant. ASSC was found in nine siblings of sickle beta-thalassaemia group, while none were found in the sickle cell anaemia group. The mean age of the first episode was significantly higher in sickle beta-thalassaemia, with significant differences in the levels of Hb F, Hb S, size of spleen and severity of crisis between both groups. In the sickle cell anaemia group the only significant difference between patients with and these without acute splenic sequestration was the difference in the size of spleen. In this study, the rate of ASSC in the sickle beta-thalassaemia patients was 32%, in contrast to 25% in the sickle cell anaemia patients. The risk of recurrence was about 70% in those who survived their first episodes. There was a close relationship between ASSC and subsequent hypersplenism. Important predictable factors for ASSC in sickle beta-thalassaemia patients were the presence of splenomegaly of more than 5 cm below the costal margin, history of acute splenic sequestration in siblings and high Hb F. Most of first episodes in sickle cell anaemia occur under the age of 2 years, while in sickle beta-thalassaemia the majority of patients have their first crisis at the age of > or =3.5 years.