Nine cases of cytomegalovirus mononucleosis in previously healthy persons were seen during a 10-month period. All were women between 19 and 28 years of age. Their illnesses were characterized by malaise, myalgia, fever, atypical lymphocytosis, and abnormal liver functions. Five patients noted sore throat, but none had exudative pharyngitis. Generalized lymphadenopathy was present in two patients and splenomegaly in three. Heterophil agglutination and ox cell hemolysin tests were uniformly negative. Seven patients had mild illnesses, one developed ascending polyneuritis (Guillain-Barré syndrome), and another had a prolonged illness complicated by splenic infarction. Persistent viruria was noted in all patients; cytomegalovirus was also recovered from oropharyngeal secretions in five patients and other sites in one patient. Cytomegalovirus mononucleosis may be common among young adults. This diagnosis should be considered in all patients with heterophil-antibody-negative mononucleosis, including those with lymphadenopathy and splenomegaly.