To determine the expression of Wilms' tumor gene (WT1) in clonal hematopoietic cells in patients with myelodysplastic syndromes (MDS), immunochemistry labelling (alkaline phosphatase anti-alkaline phosphatase) and fluorescence in situ hybridization (FISH) were coperformed on bone marrow cytospins from 18 patients whose abnormal karyotypes had been determined by G-binding analysis. Compared to 12 healthy donors, WT1 positive nucleated cells in MDS marrows were significantly higher (t = 2.30; P = 0.032). Moreover, WT1 was expressed predominantly in MDS clonal cells (with abnormal FISH signals) rather than in non-clonal cells (residual normal cells) (t = 2.19; P = 0.043).