The pathogenesis of the hyaline vascular variant of Castleman's disease is currently unknown; however, vascular and dendritic cell proliferations are common in this disorder. We report a clonal karyotypic abnormality (46,XX,t(1;16) (p11;p11), del(7)(q21q22),del(8)(q12q22)) in 15 of 20 cells obtained after short-term stromal cultures of a typical case of hyaline vascular Castleman's disease (HVCD). There was no histologic, immunohistochemical, or genotypic evidence of a clonal lymphoid or plasma cell proliferation supporting origin of this aberration from the stromal component, possibly dendritic cells. We re-examined 15 previous cases of HVCD and identified a spectrum of dysplastic changes in the follicular dendritic cells (FDC) of atrophic lymphoid follicles, with some cases showing expansions of FDC networks by CD21 immunostaining. We propose that localized clonal proliferations of stromal elements, particularly follicular dendritic cells, occur in typical HVCD and likely explain the increased incidence of FDC sarcomas in these patients.