We describe a 36-year-old man uninfected with human immunodeficiency virus who had confirmed concurrent infection with Epstein-Barr virus (EBV) and human herpesvirus 8 (HHV-8) and their respective lymphoproliferative manifestations, nasal-type NK/T cell lymphoma and Castleman disease. Antibodies to HHV-8 and EBV DNA were found in plasma and peripheral blood mononuclear cells. An EBV-positive nasal-type NK/T cell lymphoma infiltrated the splenic red pulp, whereas the white pulp contained HHV-8-positive plasmablasts, as found in Castleman disease.