Multicentric Castleman's disease (MCD) is a clinicopathologically defined entity characterized by systemic lymphadenopathy with unique pathomorphology such as angiosclerosis, blood vessel proliferation in and around follicles, and plasmacytosis. While its pathogenesis has remained unclarified for many years, identification of the human herpesvirus 8 (HHV-8) in at least some MCD cases has opened new perspectives in this field. Because previous reports have described many inconsistencies regarding HHV-8 positivity in MCD, we intended to clarify this issue by the introduction of more convincing methodologies. For this investigation, we introduced two antibodies produced in our laboratories that recognize a latent gene product ORF73 and a lytic gene product ORF59, together with two well-recognized methods, in situ hybridization for the detection of lytic phase transcript T1.1/nut-1, and genomic polymerase chain reaction (PCR). Eighty-two cases of MCD were collected from Japan (n = 75) and France (n = 7). In three cases, the patients were suffering from acquired immunodeficiency syndrome (AIDS). Immunohistochemistry and in situ hybridization showed identical results: only three out of 82 cases were positively stained, and all the positive cases were found to be the patients with AIDS. Genomic PCR was done in 43 cases, and only one case produced positive results: the only AIDS case among the 43 cases studied by genomic PCR. Histopathologically, the HHV-8-positive cases showed the highest intensity of angiosclerosis and germinal center / perifollicular vascular proliferation, while plasmacytosis was not severe in the HHV-8-positive cases. Some of the HHV-8-negative MCD cases displayed similar histopathology, but at a far less intense level, except for the plasmacytosis. These results suggest that: (i) all three of the HHV-8-positive MCD patients in the present group are the patients with AIDS; and (ii) HHV-8-positive MCD patients develop typical but marked angiosclerosis and vascular proliferation that might be differentiated from HHV-8-negative MCD patients, who showed far less intense changes.