Thiarabine has demonstrated exceptional antitumor activity against numerous human tumor xenografts in mice, being superior to gemcitabine, clofarabine, or cytarabine. Unlike cytarabine, thiarabine demonstrated excellent activity against solid tumor xenografts, suggesting that this agent has the kind of robust activity in animal models that leads to clinical utility. Thiarabine is effective orally (bioavailability of approximately 16%) and with once per day dosing: Two characteristics that distinguish it from cytarabine. Although both the structure and basic mechanism of action of thiarabine are similar to that of cytarabine, there are many quantitative differences in the biochemical pharmacology of these two agents that can explain the superior antitumor activity of thiarabine. Two important attributes are the long retention time of the 5′-triphosphate of thiarabine in tumor cells and its potent inhibition of DNA synthesis. The biochemical pharmacology of thiarabine is also different from that of gemcitabine. Thiarabine has been evaluated in three phase I clinical trials, where it has demonstrated some activity in heavily pretreated patients with hematologic malignancies and solid tumors. Because of its impressive activity against numerous human tumor xenografts in mice, its unique biochemical activity, and encouraging clinical results in phase I clinical trials, we believe thiarabine should continue to be evaluated in the clinic for treatment of hematologic and/or solid tumors. The preclinical results to date (superior in vivo antitumor activity, oral bioavailability, and once per day dosing), suggest that thiarabine could replace cytarabine in the treatment of acute myelogenous leukemia.