Drug resistance is one of the major reasons for failure of chemotherapy of acute leukemia with cytosine arabinoside (ARA-C). In order to overcome this problem we have investigated the interaction of ARA-C with 3-deazauridine (3-DU) against HL-60 myeloid leukemic cells. 3-DU is an interesting agent to use in combination with ARA-C, since drug-resistant cells that are deficient in deoxycytidine kinase are very sensitive to this uridine analogue. We have observed that for both short and long drug exposure there was a potent synergistic interaction between ARA-C and 3-DU with respect to their cytotoxic effects on HL-60 leukemic cells. This synergy could be explained by an increased cellular uptake of ARA-C to ARA-CTP by the leukemic cells in the presence of 3-DU, due to the reduction in the pool of dCTP produced by this latter analogue. Since dCTP is a potent feedback inhibitor of the phosphorylation of ARA-C by deoxycytidine kinase, the reduction in the dCTP produced by 3-DU results in an increased rate of phosphorylation of the arabinosyl analogue. Our results suggest that ARA-C and 3-DU may be an interesting drug combination to circumvent drug resistance in the chemotherapy of acute leukemia.