F10 is an oligonucleotide based on the thymidylate synthase (TS) inhibitory 5-fluorouracil (5-FU) metabolite, 5-fluoro-2'-deoxyuridine-5'-O-monophosphate. We sought to determine the activity of F10 against preclinical models of acute lymphoblastic leukemia (ALL). F10 treatment resulted in robust induction of apoptosis that could not be equaled by 100 fold more 5-FU. F10 was more potent than Ara-C and doxorubicin against a panel of murine and human ALL cells with an average IC50 value of 1.48 nM (range 0.07 to 5.4 nM). F10 was more than 1000 times more potent than 5-FU. In vivo, F10 treatment significantly increased survival in 2 separate syngeneic ALL mouse models and 3 separate xenograft models. F10 also protected mice from leukemia-induced weight loss. In ALL cells made resistant to Ara-C, F10 remained highly active in vitro and in vivo. Using labeled F10, uptake by the ALL cell lines DG75 and SUP-B15 was rapid and profoundly temperature-dependent. Both cell lines demonstrated increased uptake compared to normal murine lineage- depleted marrow cells. Consistent with this decreased uptake, F10 treatment did not alter the ability of human hematopoietic stem cells to engraft in immunodeficient mice.