CD10 is one of the hallmarks of germinal center B-cells where follicular lymphomas (FL) originate. It has not been clearly established, however, whether CD10+ diffuse B-cell lymphomas (DLBCL) are genetically similar to FL. We therefore examined 19 CD10+DLBCL and 40 FL by means of comparative genomic hybridization (CGH) and tissue-fluorescence in situ hybridization (T-FISH). Chromosomal imbalance was more frequently detected in CD10+DLBCLs (19/19) than in FLs (24/40). Significant differences were found in eight frequently imbalanced regions, namely those with gains of chromosomes 7q and 12 and those with losses of chromosomes 1p, 4p, 6q, 15q, 16p and 17. Amplification of the 3q region where BCL6 is located is reported to occur frequently in DLBCL, but it was only found in one of the 19 CD10+DLBCL cases we examined. The involvement of t(14;18) in CD10+DLBCL (31%) and in FL (73%) was significantly different (P=0.0064). The CGH pattern of CD10+DLBCL with t(14;18) was also different from that of FL with t(14;18). Taken together, our results indicate that CD10+DLBCL constitutes a unique subtype entity with genetic characteristics significantly different from those of FL and DLBCL.