Two patients developed fever, interstitial pneumonitis, and pancytopenia associated with extremely high titers of antibody to replicative antigens of the Epstein-Barr virus. In contrast to most patients seropositive for Epstein-Barr virus, neither patient had an antibody response to the Epstein-Barr nuclear antigen K polypeptide. In addition, virus isolated from one patient had a deletion of the B95-8 type in the EcoRI C region of the genome. An etiologic relation between Epstein-Barr virus replication and the clinical manifestations of this syndrome is further shown by the response of each patient to acyclovir therapy. These patients have a new Epstein-Barr-virus-associated syndrome and provide additional evidence that acyclovir may play a role in therapy for selected patients with Epstein-Barr virus infection.