People who have undergone splenectomy mount a poor IgM response to bacterial polysaccharide vaccines. Whether this defect is true during natural bacterial and viral infections is unknown. We present 2 cases of postsplenectomy cytomegalovirus (CMV)-induced mononucleosis with impaired IgM but normal to augmented IgG response. The cases presented initial diagnostic challenges owing to a prolonged course of infection, marked lymphocytosis (peak lymphocyte count, 27,900/microL [27.9 10(9)/L]), clonal T-cell proliferation with T-cell receptor g gene rearrangements, and remote history of splenectomy. However, the acute nature of the infections, serial determinations of the anti-CMV IgM and IgG, exclusion of other causes, and detection of CMV in the blood established the diagnosis and revealed the deranged antibody response. The infections resolved without specific treatment. These cases suggest that the spleen might be a primary site for specific anti-CMV IgM response.