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To analyze the genomic differences between multiple myeloma (MM) and plasma cell leukemia (PCL), a total of 30 cases were studied by comparative genomic hybridization (CGH). In five cases with a low proportion of plasma cells (PC) in bone marrow, an enrichment of PC was performed by using immunomagnetic beads conjugated with the monoclonal antibody B-B4. In 24 out of the 25 MM (96%) and in all five PCL (100%) patients DNA copy number changes were identified by CGH analysis; in the MM case without chromosomal imbalances, the immunomagnetic enrichment of PC had failed. The most recurrent changes in MM patients were gains at chromosomes 15q (48%), 11q (44%), 3q (40%), 9q (40%) and 1q (36%). By contrast, all PCL patients showed gains in 1q. Losses of chromosomal material were significantly more frequent in PCL than in MM patients (P = 0.03): losses on 13q in 80% of PCL vs 28% of MM; and on chromosome 16 in 80% vs 12%, respectively. In addition, PCL patients showed losses of 2q and 6p that were not present in MM. The CGH data show differences in chromosomal imbalances between MM and PCL.