The plasma cell variant of Castleman's disease (angiofollicular hyperplasia) is an uncommon and poorly understood lymphoproliferative disorder which may be associated with a wide variety of systemic features. The clinical, pathological, immunophenotypic and genotypic features of a series of five cases are presented. The clinical manifestations were variable but systemic features were present in four cases. Histologically, all five cases were similar. The characteristic follicle-like structures are composed of concentric rings of B-lymphocytes with a mantle zone phenotype, surrounding an inner core of dendritic reticulum cells. The central regions are composed of plump cells that express Factor VIII related antigen, stain with Ulex europaeus I and are associated with laminin immunoreactivity. Many of these cells stain with Ki67 indicating cellular proliferation. Between these 'follicles', plasma cells are present interspersed between high endothelial venules. Using immunohistochemical techniques two cases were polyclonal, whilst in two others unequivocal lambda/IgA restriction was present; in the other case an intermediate picture of lambda/IgA predominance was apparent. Genotypic studies in three cases indicated immunoglobulin gene rearrangements but germ line T-cell receptor genes. The significance of the apparent clonal proliferation of plasma cells is discussed and a unifying model of Castleman's disease is proposed.