Infectious mononucleosis is caused by the Epstein-Barr virus (EBV) and most commonly affects young adults from 15 to 35 years of age. The diagnosis is made by accurate assessment of clinical, hematologic and serologic manifestations of the illness. Manifestations include the classic triad of fever, pharyngitis and cervical lymphadenopathy; lymphocytosis with a predominance of atypical lymphocytes; a positive heterophil (Monospot) antibody test; and in some cases, serologic evidence of EBV-specific antibodies produced against antigens related to the virus. The most valuable serologic finding is the presence of IgM antibody to EBV viral capsid antigen, which is found during acute primary EBV infection. Infectious mononucleosis is considered a self-limited illness, but it may result in serious complications involving the pulmonary, ophthalmologic, neurologic and hematologic systems. Treatment is focused on managing the symptoms, unless more severe disease involving other organ systems occurs. The most common potentially fatal complication is splenic rupture.