Two-hundred and twenty-one bone marrow biopsies with lymphoid infiltrates were studied histologically and immunohistochemically, to assess the incidence and the pattern of follicular dendritic cells. Three monoclonal antibodies selective for follicular dendritic cells were used: CD21, CD35 and DR53, all reactive on paraffin-embedded material. Follicular dendritic cells were present in two of 38 benign lymphoid aggregates, 92 of 134 low grade B-cell lymphomas (45 of 62 lymphocytic, 16 of 27 lymphoplasmacytoid, 0 of six hairy cell leukaemias, five of six centrocytic, 19 of 21 centroblastic-centrocytic, seven of 12 low grade NOS), one of 23 high grade B-cell lymphomas, 0 of 10 T-cell lymphomas, 0 of three Hodgkin's disease and four of 13 suspicious infiltrates. Follicular dendritic cells were found in lymphomatous involvement with nodular, patchy and massive growth pattern, but not in interstitial ones. They formed follicle-like networks, whose number and size were directly correlated to the tumour mass. The origin and frequency distribution of follicular dendritic cells in bone marrow biopsy lymphomas is discussed and the diagnostic relevance of follicular dendritic cell immunostaining in routine bone marrow biopsy lymphoid infiltrates is assessed