Total and differential WBC counts were measured in 88 children with sickle cell hemoglobinopathies during the steady state, vaso-occlusive crisis, and bacterial infection. Steady state leukocyte values were lower than anticipated on the basis of currently available information. Total and segmented leukocyte numbers were greatly increased during vaso-occlusive crisis and infection, but only with bacterial infection was there a consistent increase in bands or nonsegmented leukocytes (mean, 4,580/microliter). On the basis of these data we believe that total and differential leukocyte counts are of value for identifying those children with potentially serious bacterial infections.