A woman with mycosis fungoides treated by psoralen with ultraviolet A (PUVA) and electron beam therapy developed acute myeloid leukemia (AML) three years later. Karyotypic analysis of the leukemia cells revealed monosomy 7. Fluorescence in situ hybridization showed that the monosomy 7 clone had accounted for about a third of the marrow cells after PUVA treatment, but replaced the entire marrow at leukemic transformation. These findings were consistent with a secondary AML evolving from an underlying myelodysplasia, supporting that PUVA therapy might have a mutagenic effect on hematopoietic cells. This might be related to its effect on circulating hematopoietic stem cells.