We investigated whether dendritic cells (DCs) play a role in favoring granular lymphocyte (GL) proliferation in patients with lymphoproliferative disease of granular lymphocytes (LDGL). The presence of in vivo circulating DCs was studied in 11 patients (5 CD3+ and 6 CD3- LDGL). Autologous immature (iDCs) and mature (mDCs) DCs generated in vitro were studied for stimulatory activity on cell proliferation of CD3+ and CD3- GLs. The topographic organization of GLs and DCs was also studied in bone marrow (BM) biopsies. Peripheral blood (PB) CD3- GLs from patients showed significant proliferative activity in the presence of iDCs and mDCs. Conversely, monoclonal CD3+ GLs were unresponsive to autologous and allogeneic PB DCs. Analysis of BM biopsies demonstrated a topographic distribution of DCs and GLs that indicates contact between the 2 cell types. On functional assays, DCs obtained from BM were more efficient than PB DCs in stimulating CD3- GLs, and surprisingly, a low but definite stimulatory effect was demonstrated also on CD3+ GLs. The putative contact between DCs and GLs in the BM and, more crucial, the proliferative response of discrete GL populations to DC stimulation suggest the presence of a specific antigen within BM DCs, providing evidence for a role of DCs in the pathogenesis of LDGL.