Multiple myeloma (MM) is a B-cell malignancy characterized by clonal expansion of plasma cells producing monoclonal immunoglobulins. It has been regarded as a tumor typically involving only the bone marrow. The existence of circulating tumor cells has been suggested from phenotypic and genotypic studies. However, this issue is still controversial due to the limitations of the methods so far used. We describe a novel polymerase chain reaction (PCR) based method using clone-specific immunoglobulin heavy-chain gene sequences as tumor markers. From such sequences patient-specific oligonucleotide primers and probes were generated, and used to detect tumor cells. Seven MM patients were selected for this study, and tumor cells were found in all peripheral blood samples. The demonstration of circulating tumor cells suggests some caution when using peripheral blood for autograft procedures, even though its contamination is lower than bone marrow. In conclusion, we describe a specific and sensitive PCR-based method for detecting minimal disease which is of general applicability to all lymphoid malignancies transcribing rearranged immunoglobulin heavy-chain genes.