We present three cases with presumptive evidence of mantle cell lymphoma (MCL) that were submitted for cytogenetic evaluation. Chromosome analysis showed a normal karyotype in two cases, while the third case showed the composite karyotype; 45,XY,t(1;22)(p13;q13),23,del(10)(q22),add(15)(q22),add(17)(p13). The characteristic t(11;14)(q13;q32) for MCL was not observed by conventional karyotyping in any of the cases. We furthermore evaluated the specimens by fluorescence in situ hybridization (FISH) using the dual-color LSI IgH/CCND1 DNA probe. Fusion signals, consistent with t(11;14)(q13;q32), were observed in 65% and 85% of interphase cells in cases 1 and 2, respectively, while the metaphases from both cases revealed a normal pattern. All abnormal metaphases as well as 57% of interphase cells from case 3 displayed a fusion signal. In the abnormal metaphase cells, the fused signal was located on the normally looking 14q32, suggesting that the IgH/CCND1 fusion resulted from the insertion of the CCND1 gene into 14q32 adjacent to the IgH gene. Thus, FISH confirmed the diagnosis of MCL by showing the IgH/CCND1 fusion. In addition, these findings indicate that the sensitivity of FISH is superior to that of conventional cytogenetics in detecting t(11;14)(q13;q32) associated with MCL.