This study investigated the useful morphologic and immunophenotypic findings for the diagnosis of Castleman's disease (CD). We focused on the distribution and expression of follicular dendritic cells (FDC) in lymphoid follicles from patients with CD. Eleven CD cases of the hyaline vascular (HV) variant and six cases of the plasma cell (PC) variant were studied using tissue microarray and paraffin resistant monoclonal antibodies CD21, CD35, and EGFR, a new novel marker of FDC, as well as an antibody against human herpes virus 8 (HHV8). Epstein-Barr virus (EBV) was detected by means of in situ hybridization with a fluorescein isothiocyanate-labeled EBV-encoded RNA (EBER) specific oligonucleotide. The FDC network of the PC variant (n=4) was similar to that seen in normal or reactive germinal centers. In contrast, all HV variants and 2 cases of the PC variant were either expanded, disrupted, or exhibited multiple tight collections of FDC both in germinal centers and in mantle zone lymphocytes. The expanded mantle zone lymphocytes were CD20+, Bcl2+, PAX5+, and MUM1- with less number of CD3+ T cells admixed. Other features of the HV variant included follicular regression and vascular ingrowth of the germinal centers, whereas features of the PC variant were follicular hyperplasia and interfollicular plasmacytosis. In addition, EBV infection was positive in three CD cases, and one case had co-expression of HHV8 and EBV infection. Taken together, we found immunophenotypic differences of mantle zone lymphocytes and FDC network patterns of lymphoid follicles in CD. Thus, we conclude that these differences are relevant for the differential diagnosis of the two histopathologic variants of CD.