Castleman's disease of the plasma cell variant observed in 21 patients was investigated morphologically and by immunohistochemistry. The male to female ratio was 2.5:1. The age ranged from 18 to 71 years (mean age 47.6 years). Histologically, the lesions were characterized by numerous, evenly distributed germinal centres ranging from large hyperplastic to small hyaline-vascular types with moderate to extensive sheets of plasma cells in between. In all cases with generalized lymph node enlargement dilated sinuses could be found. Immunohistochemical studies demonstrated a polyclonal plasma cell population in 11 of 18 lesions. Seven lesions contained a monoclonal plasma cell population, five with IgG/lambda and two with IgA/lambda; clinically, two of these patients had generalized lymphadenopathy; in none could manifestation of a plasmacytoma be found outside the lymph node lesion; only two of the seven patients exhibited a paraproteinaemia (IgG/lambda and IgA/lambda, respectively) corresponding to the Ig type of the proliferated plasma cells. Six patients with polyclonal lesions had constitutional symptoms such as fever, anaemia, polyclonal hypergammaglobulinaemia; one of these six patients manifested symptoms consistent with a diagnosis of Takatsuki syndrome. The monoclonal plasma cell type of Castleman's disease did not progress to a generalized disease. This monoclonal variant may be a lymph nodebased type of benign monoclonal gammopathy.