Hyperdiploidy sometimes is found in mycosis fungoides-Sézary syndrome, but its diagnostic significance remains undefined. We report an unusual case of Sézary syndrome manifesting with leukemic large cell transformation. Conventional karyotypic analysis showed the presence of a near-tetraploid neoplastic clone. With dual-color cytometric analysis, we showed that the large Sézary cells were near-tetraploid with a DNA index of 1.86, thereby demonstrating a direct relationship between cell size and ploidy. Comparative genomic hybridization further showed chromosomal imbalances that were not revealed on conventional karyotyping. Our findings suggest that hyperdiploidy may be a marker of large cell transformation, so that when this karyotypic abnormality is found in mycosis fungoides-Sézary syndrome, a search for such a complication is indicated.