Multiple myeloma is characterized by the proliferation of a monoclonal plasma cell population that essentially spreads in the bone marrow. However, immunological data and studies on clonal gene rearrangements have provided evidence for circulating malignant B-lymphocytes. Herein, we report on the detection and clinical significance of monoclonal lymphocytes in the peripheral blood of multiple myeloma patients. Applying the analysis of immunoglobulin gene rearrangements, clonal circulating lymphocytes were found in 8 out of 37 patients. In three of these eight patients, DNA from the bone marrow could be examined as well, and the same gene rearrangement as in peripheral blood was detected. This observation indicates that the monoclonal lymphocytes in the peripheral blood are part of the malignant plasma cell clone in the bone marrow. There was a strong correlation between progressive stage III disease and the detection of circulating tumour cells, whereas neither in stable stage III nor stage I disease could clonal gene rearrangements be detected. Our findings indicate the high incidence of monoclonal lymphocytes in the peripheral blood from patients with progressive multiple myeloma. This may be of importance with respect to cell harvesting strategies for autologous peripheral stem cell transplantation in multiple myeloma.