Most of the circulating lymphocytes from three asymptomatic adults (one male, two female, age range 61-67 years) with isolated persistent lymphocytosis of between 7.1 and 10 x 10(9)/l possessed characteristic villous projections of the cell membrane. Morphological, histochemical, ultrastructural, immunological, and genotypic studies confirmed a clonal proliferation of tartrate-resistant acid phosphatase (TRAP)-negative CD5-CD10-CD25- and CD11c+ B-cells. In addition to CD11c, these cells expressed other adhesion receptors (LFA-1/CD11a, VLA-4/CD29/49d, ICAM-1/CD54, and LAM-1) and produced detectable amounts of interleukin-1 beta, interleukin-6, and in one case tumour necrosis factor-alpha mRNA. This monoclonal villous lymphocytosis (MVL) could be differentiated from B-cell chronic lymphocytic, prolymphocytic, and hairy cell leukaemias, and from previously recognized CD11c+ chronic B-cell leukaemia. A rare splenomegalic non-Hodgkin's lymphoma variant with circulating villous B-lymphocytes (SLVL), usually CD10+ and sometimes CD11c- and TRAP+, appears to be a closely related disorder. In all three patients the lymphocyte count increased very slowly, at a rate less than 5 x 10(9)/l per year, over 3-7.5 years of follow up, and a moderate splenomegaly eventually developed in one of the patients. Chemotherapy was never required. MVL may be a relatively benign clinical entity akin to SLVL within the group of CD11c+ B-cell lymphoproliferative disorders.