Eight children with congenital asplenia syndrome have been studied for their cardiac and immunologic status. All patients were greater than 2 years of age and had severe complex cyanotic heart disease. All eight patients had abnormalities of cardiac and/or visceral situs. All patients had evidence of pulmonary stenosis or atresia and a common atrium or large atrial septal defect. Five patients required palliative cardiac surgery. All patients were given prophylactic antibiotics; there were no documented episodes of sepsis. One patient had an isolated deficiency of IgM; two patients had an isolated deficiency of IgE. Seven of eight patients were immunized with a dodecavalent pneumococcal vaccine. Four of the seven patients failed to have a twofold or greater antibody response. Our findings suggest that prophylactic antibiotics may reduce the incidence of sepsis in the asplenia syndrome. Because the prognosis for these patients must be optimistic, we recommend early documentation of splenic function in children suspected of having the asplenia syndrome, prophylactic antibiotics, and parent education. Children immunized with bacterial vaccines should have their antibody responses monitored.