Reed-Sternberg (RS) and Hodgkin's (H) cells are considered to be the neoplastic cells in Hodgkin's disease (HD). Although most data suggest their lymphoid origin, the nature of these cells still remains a subject of controversy. Recently, a number of RS cells have been found to express an antigen that is also present on follicular dendritic cells (FDCs), asserting FDCs as the possible progenitor cells of H-RS cells. This prompted us to investigate whether these CD21-positive cases had distinct clinicopathologic characteristics. In a series of 94 examined cases of HD, we identified 9 CD21-positive ones (4 of 37 cases of nodular sclerosis, 1 of 41 mixed cellularity, and 4 of 12 lymphocyte depletion HD) without any other B-cell marker on paraffin sections. The patients varied in age from 16 to 82 years (median, 50 years) and included six men and three women. They had superficial or mesenteric lymphadenopathy without hepatosplenomegaly. Peripheral blood leukocytosis was seen in three patients. The clinical course was indolent, and all patients but one achieved an initial complete response with HD-based treatment regimens, although three of them relapsed. Morphologically, two subgroups could be delineated. Six of the cases were characterized, besides by the classic RS cells, by a varying number of the cells with the distinctive walnutlike or cerebrumlike nuclei and cytologically with cytoplasmic processes. Their fine structural examination also revealed villous processes, but no desmosomes. The other three cases had multinucleated RS cells often with triangular nuclei, but not cytoplasmic processes. The percentage of CD21-positive tumor cells ranged from less than 10% to 60% among the H-RS cells. These RS cells were positive for CD30 (9 of 9), CD15 (7 of 9), CD68 (1 of 8), fascin (8 of 8), S-100 protein (1 of 7), and epithelial membrane antigen (2 of 8) on paraffin sections. Notably, of eight cases examined on frozen sections, two showed immunostaining for DRC1, CD35, R4/23, and Ki-M4p. Only CD35 was also detected in the other two cases. Genotypic investigation showed germline configuration of the T-cell receptor beta and gamma chain genes and the immunoglobulin heavy chain gene in all eight cases examined. In situ hybridization showed Epstein-Barr virus sequences in four cases, three of which were examined by the terminal region analysis and showed the Epstein-Barr virus to be monoclonal. We concluded that in a small proportion (9.6%) of HD, H-RS cells might be derived from FDCs and that they appear to represent a distinct pathologic variant based on morphologic and phenotypic traits within the framework of HD.