Of a cohort of 308 children with homozygous sickle cell disease diagnosed at birth, 89 experienced 132 clinically significant attacks of acute splenic sequestration (ASS) over a 10-year period. The age at first attack ranged from 3 months to 6 years. Survival curve analysis of the interval until first attack indicated a cumulative probability of 0.225 by 2 years, and 0.265 by 3 years, and 0.297 by 5 years of age. Thirteen events were fatal, 11 during the first attack, and all before transfusion could be instituted. Recurrences occurred in 49% of survivors of the first attacks, and there were diminishing intervals between subsequent events. Respiratory symptoms were associated with 52 of 132 events, but bacterial isolates on blood culture were less frequent, and ASS was not prevented by pneumococcal vaccine or penicillin prophylaxis. A high fetal hemoglobin level protected against attacks of ASS. A parental education program aimed at early diagnosis of ASS was followed by an increase in the incidence rate for ASS from a mean of 4.6 per 100 patient-years to 11.3 per 100 patient-years, probably reflecting increased awareness of the complication. During the same periods, the fatality rate fell from 29.4 per 100 events to 3.1 per 100 events. The improvement in outcome is likely to have resulted from improvement in medical management and earlier detection of ASS.