Infectious mononucleosis is an acute, self-limiting, nonneoplastic lymphoreticular proliferative disorder characterized by peripheral lymphocytosis and circulating atypical lymphocytes. Epstein-Barr virus is the causative agent in 90% of cases. Highest incidence is in the 15− to 25− year-old age-group, with 1% to 3% of all college students in the United States affected each year. Clinical manifestations vary according to age at presentation. Incubation period is 4 to 7 weeks. Diagnosis is primarily made with the monospot test but may include throat culture and complete blood count with differential. Cytomegalovirus and human immunodeficiency virus are among the many other conditions that may present initially as infectious mononucleosis. Treatment is supportive with prevention of complications as the goal; good personal hygiene and avoidance of contact sports should be stressed.