Several studies have reported previously that acute myeloid leukemia (AML) may express WT1 detected by RT-PCR and/or Northern blotting. The diagnostic utility of WT1 expression in AML using immunohistochemistry has not been reported previously. Paraffin-embedded tissue sections from 55 AML, 12 acute lymphoblastic leukemia (ALL), and 10 normal bone marrow specimens were immunostained for WT1 (anti-N terminus antibody). 22/55 AML cases (40%) demonstrated nuclear immunopositivity for WT1, including 20/47 bone marrow trephines and 2/4 granulocytic sarcomas. All the ALL and normal bone marrow specimens were negative. A significant proportion of AML expresses nuclear immunostaining for WT1, a finding that has only been described previously in Wilms' tumor and desmoplastic small round cell tumor. This finding is important for the correct interpretation of immunohistochemical findings in the diagnosis of "small round cell'' tumors of childhood, especially in cases of extramedullary deposits of AML, in which traditional myeloid markers may be negative.