Little is known about the molecular or cytogenetic alterations of mycosis fungoides (MF) large cell transformation. We report our findings on chromosomal rearrangement, based on peripheral blood and skin examination before and after cutaneous MF large cell transformation, using both conventional and molecular cytogenetic techniques. Blood cells exhibited a similar hypodiploid karyotype before and after MF transformation. A near-tetraploid karyotype with complex structural rearrangements was established from a skin tumor after MF large cell transformation. Both recurrent chromosome abnormalities and an identical T-cell receptor gamma-chain rearrangement were shared by blood and skin cells, suggesting that MF large cell transformation derived from a common monoclonal ancestor detected at MF stage. A complex hypotetraploid karyotype was established only from the skin tumor, however. MF large cell transformation may be associated with chromosome duplication followed by chromosome losses and interchromosomal rearrangements. Accordingly, additional parallel blood and skin tumor cytogenetic studies are required to further identify the recurrent cytogenetic changes associated with the aggressiveness of the disease after large cell transformation.