In adult acute myeloid leukemia (AML), the weight of the contribution of the combined activity of Pgp and MRP1 to drug resistance is not known. To address this question, we compared the activity of these proteins to the in vitro resistance to daunorubicin (DNR), etoposide, and cytosine arabinoside (Ara-C), using the calcein-AM uptake and the 3-[4, 5-di-methyl-thiazol-2, 5-diphenyl] tetrazolium bromide (MTT) assay in 80 adult AML patients. We found no correlation or only a weak correlation between the in vitro drug resistance to DNR and etoposide and MRP1 or Pgp expression or function when tested separately. However, a strong correlation was observed between the simultaneous activity of MRP1 and Pgp (quantified as the modulation of calcein-AM uptake by cyclosporin A and probenecid) and the LC50 of DNR (r =.77, P <.0001). This emphasized the role of these two proteins, not separately, but together in the resistance to DNR. In contrast, Mvp/LRP expression did not correlate with the LC50 of DNR. A high level of simultaneous activity of Pgp and MRP1 was predictive of a poor treatment outcome (for achievement of CR [P =.008], duration of relapse-free survival [RFS; P =.01], and duration of overall survival [OS; P =.02]). In addition, high LC50 of DNR and high LC50 of etoposide together were also predictive of a poor treatment outcome (for duration of RFS [P =.02] and duration of OS [P =.02]). The unfavorable cytogenetic category was more closely associated with the combined activity of both MRP1 and Pgp (P =.002) than with the activity of Pgp or MRP1 separately. This could explain the poor prognosis and the in vitro resistance to daunorubicin in this group of patients. These data suggest that treatment outcome may be improved when cellular DNR and etoposide resistance can be circumvented or modulated. Modulation of not only Pgp but also MRP1 could be essential to attain this aim in adult AML.