Multicentric Castleman's disease (MCD) is an atypical lymphoproliferative disorder defined using clinical and pathologic criteria. A characteristic of the MCD is a close association with Kaposi's sarcoma (KS), which occurs during the clinical course of most human immunodeficiency virus (HIV)-associated MCD cases and also, but less frequently, in HIV-negative patients. Recently, sequences of a putative new Herpesvirus (KSHV) have been isolated and further detected in almost all the acquired immunodeficiency syndrome (AIDS) KS and in most of the non-AIDS KS samples. In this study, we searched for these Herpesvirus-like sequences in MCD samples of 31 patients. KSHV sequences were detected in 14 of 14 cases of HIV-associated MCD, including 5 cases without detectable KS. Moreover, KSHV was detected in 7 of 17 MCD cases in HIV-negative patients, including 1 case associated with a cutaneous KS. In 34 non-MCD reactive lymph nodes (follicular and/or interfollicular hyperplasia) in HIV-negative patients, KSHV was detected in only 1 case. In 1 HIV-negative case of MCD, KSHV was found in both the lymph node and peripheral blood samples. These data suggest that KSHV could play a role in the pathogenesis of MCD, especially in HIV-infected patients.