Infectious mononucleosis should be suspected in patients 10 to 30 years of age who present with sore throat and significant fatigue, palatal petechiae, posterior cervical or auricular adenopathy, marked adenopathy, or inguinal adenopathy. An atypical lymphocytosis of at least 20 percent or atypical lymphocytosis of at least 10 percent plus lymphocytosis of at least 50 percent strongly supports the diagnosis, as does a positive heterophile antibody test. False-negative results of heterophile antibody tests are relatively common early in the course of infection. Patients with negative results may have another infection, such as toxoplasmosis, streptococcal infection, cytomegalovirus infection, or another viral infection. Symptomatic treatment, the mainstay of care, includes adequate hydration, analgesics, antipyretics, and adequate rest. Bed rest should not be enforced, and the patient's energy level should guide activity. Corticosteroids, acyclovir, and antihistamines are not recommended for routine treatment of infectious mononucleosis, although corticosteroids may benefit patients with respiratory compromise or severe pharyngeal edema. Patients with infectious mononucleosis should be withdrawn from contact or collision sports for at least four weeks after the onset of symptoms. Fatigue, myalgias, and need for sleep may persist for several months after the acute infection has resolved.