In three patients with infectious mononucleosis the disease was severely complicated by, respectively, aplastic anemia, consumptive thrombocytopenia and hemolytic anemia with acute renal failure. In contrast to the regularly recurring basic signs and symptoms of the illness, serious complications occur in less than 1% of all patients. Changes in the immunologic system seem to play an important role in hematologic and other complications, as well as in the occasionally fatal Epstein-Barr virus infection. Pathophysiological, and particularly immunologic mechanisms, and possible therapy are discussed with reference to case reports of patients with similar clinical courses and the literature on infectious mononucleosis.