Multidrug resistance (MDR) phenotype expression was evaluated retrospectively in 87 patients with acute myeloid leukemia (AML), 69 with de novo AML, ten with relapsed AML and eight with AML secondary to myelodysplastic syndrome (MDS). MDR phenotype, characterized by P-glycoprotein expression (MRK16 monoclonal antibody) and decrease in intracellular daunorubicin (DNR) accumulation was determined using flow cytometry. All patients received chemotherapy including cytosine-arabinoside and anthracycline (daunorubicin, zorubicin, idarubicin) or mitoxantrone, and quinine in ten cases. The predictive value of the MDR phenotype for clinical responsiveness was studied using uni- and multivariate analyses. Univariate analysis showed that DNR accumulation (p < 10(-4)), P-glycoprotein expression (p = 10(-4)) and disease status (de novo versus recurrent AML and acute MDS) (p = 10(-4)) were predictive of clinical responsiveness. The significance of these three parameters was maintained in multivariate analysis. When de novo AML was considered, only DNR accumulation was of predictive value (p < 10(-4)) for complete response to chemotherapy.