The overexpression of a cell-surface glycoprotein termed P-glycoprotein (P-gp) is frequently associated with multi-drug resistance (MDR) in cell lines in vitro. To evaluate the implications of P-gp expression in clinical drug resistance, the authors examined the expression of P-gp in leukemia cells from patients with acute myelogenous leukemia (AML) and those with acute lymphoblastic leukemia (ALL) at initial presentation and relapse, using immunoblotting with a monoclonal antibody against P-gp, C219. Nine of 17 patients with AML and four of 11 patients with ALL had P-gp-positive results at the initial presentation, and most P-gp-positive patients did not respond to chemotherapy. Four of seven patients at the relapsed stage and all three patients with preceding myelodysplastic syndrome had P-gp-positive results. The expression of P-gp and clinical refractoriness to chemotherapy were highly correlated. These data indicate that the expression of P-gp is closely related to clinical drug resistance in acute leukemia.