The expression of the MDR1 gene, a multidrug resistance gene, was determined in patients (N = 24) with myelodysplastic syndrome or acute myeloid leukemia evolving from it. MDR1 RNA expression of the mononuclear cells was detected in 14 (58%) patients. Among patients who had progressed to acute myeloid leukemia (sAML) (N = 14), MDR1 RNA expression was seen in 8 (57%) patients. Expression was observed in the 2 patients who had been pretreated with MDR drugs. These findings indicate that the MDR1 gene is frequently expressed in MDS or sAML and suggest that multidrug resistance might be involved in the clinical drug resistance of these diseases.