A 14-year-old boy with hemoglobin SC disease and alpha-thalassemia-2 experienced five episodes of acute splenic sequestration crisis (ASSC), while two of his siblings with identical globin genotypes (SC and -alpha/alpha alpha) had no such experience. To determine if an additional red blood cell (RBC) defect was responsible for the unusual occurrence of frequent ASSCs, we performed detailed rheologic characterization and membrane protein analysis on RBCs from the proband and other members of his family. Reduced surface area, increased mechanical instability, and decreased spectrin content of the membrane, distinguishing features of RBCs in hereditary spherocytosis, were observed in cells from the proband and his mother, but not in cells from other family members. These findings are consistent with the dominant inheritance of spherocytosis by the proband. We suggest that the combined effects of SC disease and spherocytosis in the proband resulted in decreased RBC deformability and led to increased splenic trapping, intrasplenic sickling, and consequently, recurrent sequestration crisis. Marked clinical and hematologic improvement occurred from splenectomy. Thus, inheritance of interacting genetic defects, sickling hemoglobinopathy, and hereditary spherocytosis appear to be responsible for the unusual clinical manifestation of recurrent ASSC in this patient.