One-hundred-sixteen consecutive bone-marrow biopsies were taken from 76 patients with essential mixed cryoglobulinemia type II (type II cryo), whose median follow-up was 97 months. Fifty-four out of fifty-six subjects who underwent ELISA and RIBA tests for HCV, were found to be positive. At conventional light microscopic examination, 64/76 patients showed discrete lymphoid infiltrates consisting of small elements with plasmacytoid differentiation and with frequent paratrabecular location. Thirty-nine biopsies were studied by immunohistochemistry that revealed the B-cell nature of the infiltrates (CD20+, CD45RA+, CD79 alpha+, CD3-, CD45RO-), with demonstrable monotypic Ig light-chain restriction in 22 cases. It is worthy of note that the lymphoid elements usually appeared protected against apoptosis, because of the strong expression of the bcl-2 oncogene product, and provided with a very low proliferative capacity, the Ki-67 index being lower that 3%. The latter findings are in keeping with the indolent behaviour of the clonal lymphoid population observed in type II cryo and allow some speculation as to the need for environmental stimuli for its maintenance as well as further mutagenic events for its eventual transformation into an overt lymphoma.