Among 20 cases of typical splenic marginal zone lymphoma (SMZL), two cases had blastic transformation. The genetic mechanisms underlying the morphologic transformation were investigated by comparing genetic changes in initial and blastic phases. A complex karyotype including trisomy of 3q and genomic gain of 17q22-q24 was seen in both cases at diagnosis. However, the extra copy of 3q was lost during the transformation process in both tumors. Additionally, the Karpas 1718 cell line, which was derived from a patient with transformed SMZL and carried a trisomy of 3q, also evidenced the spontaneous loss of the extra 3q during the culturing process. Other acquired abnormalities observed exclusively in the transformation phase included amplification and/or translocation of bands 7p22-q22 and 19p13. These findings suggest that the loss of + 3q and the acquisition of other genomic imbalances may represent unique markers for the transformation process of SMZL. We hypothesize that the trisomy of 3q may correlate with the indolent nature of SMZL, and that the loss of this acquired abnormality leads to or accompanies the development of blastic tumors.