The presence of circulating neoplastic cells at diagnosis was assessed in the blood of patients presenting with mantle cell lymphoma (MCL) to determine the feasibility of a diagnostic molecular assay. Blood samples from 16 patients with pathologically reviewed MCL were analysed for the t(11;14)(q13;q32) translocation by the polymerase chain reaction (PCR): 7 (44%) were found positive. The remaining cases were examined by PCR for the presence of circulating neoplastic B-cells by amplifying the third complementarity region (CDR3) of immunoglobulin heavy chain genes and the immunoglobulin light kappa chain deletion rearrangements. A further 7 (44%) patients showed the presence of clonal lymphoma cells, leaving only 2 (12%) of cases negative for circulating lymphomatous cells. This study suggests that there is a high incidence of lymphoma cells in the blood of patients presenting with MCL. PCR for these clonal cells may be diagnostically useful.