Multicentric Castleman's disease (MCD), also called multicentric angiofollicular lymphoid hyperplasia, is a systemic lymphoproliferative disorder causing fever, lymphadenopathy and splenomegaly. Recently, Kaposi's sarcoma-associated herpesvirus/human herpesvirus 8 (KSHV/HHV-8) DNA sequences have been detected in cases of MCD. We examined HHV-8 DNA sequences in the peripheral blood mononuclear cells (PBMCs) of two HIV-negative patients with MCD and in PBMCs and the lymph node of a HIV-negative patient with localized Castleman's disease (LCD) by the polymerase chain reaction. The novel sequences were detected in all DNA samples. Furthermore, the sequences were detected in only the CD19+ B-lymphocyte fraction of the patient with LCD as previously reported. However, the sequences were detected in CD19+ B-lymphocyte and CD2+ T-lymphocyte fractions of two patients with MCD. These results suggest that HHV-8 has tropisms for both B lymphocytes and T lymphocytes in Castleman's disease.