Follow-up of the newborn cohort provided data demonstrating that life-threatening septicemia can occur as early as four months of age, and that the incidence of septicemia in children with sickle cell disease is higher than previously reported. Serial measurements of pocked or vesiculated red blood cells determined the onset of splenic dysfunction. The loss of splenic function has been characterized by genotype and the observed patterns of loss of splenic function were different developmentally depending on the sickle hemoglobinopathy syndrome.