Podocalyxin is a CD34 family member expressed by podocytes, vascular endothelium, mesothelium, and a subset of hematopoietic progenitors. Podocalyxin expression was not observed in the hematopoietic cells of normal adult bone marrow samples. However, podocalyxin was expressed by blasts in 30 (77%) of 39 cases of acute myeloid leukemia (AML), 22 (81%) of 27 cases of acute lymphoblastic leukemia (ALL), and 13 (87%) of 15 cases of cutaneous myeloid sarcoma. No correlation with CD34 expression by immunohistochemical analysis was seen. Wilms tumor 1 (WT1) expression was detected in blasts in 17 AML cases (44%) and 21 ALL cases (78%). There was no correlation between WT1 and podocalyxin expression. We conclude that podocalyxin is expressed commonly by blasts in ALL and AML. Analysis of the expression of CD34 and podocalyxin increases sensitivity for the immunophenotypic detection of leukemic blasts compared with the analysis of CD34 alone. Therefore, podocalyxin seems to complement CD34 as a useful hematopoietic blast marker. The physiologic role of podocalyxin in leukemic blasts remains unknown.