Both oral and intravenous acyclovir administration for seven days in the early stages of infectious mononucleosis caused an inhibition of oropharyngeal Epstein-Barr virus (EBV) replication. Minimal effect on clinical symptoms was observed. Development of normal cellular and humoral EBV-specific immunity was seen in all patients. The combination of intravenous acyclovir and prednisolone treatment for 10 days in 11 patients with fulminant mononucleosis caused transient cessation of virus shedding in all patients. A dramatic clinical effect on pharyngeal symptoms and on fever was seen in nine of 11 patients within 72 hours. Treatment with chemotherapy or irradiation is recommended in EBV-associated B cell lymphomas seen in immunosuppressed, transplanted, and human immunodeficiency virus-I seropositive patients. No effect of acyclovir has been reported, but such therapy may be considered in the early stage when EBV induces a polyclonal B cell activation. Acyclovir treatment is effective in the EBV-genome positive hairy leukoplakia in human immunodeficiency virus-seropositive patients. No effect of antiviral therapy has been reported in the X-linked lymphoproliferative syndrome. Prophylactic use of immunoglobulin or acyclovir has been suggested in susceptible children.