We demonstrate here that cytarabine and daunorubicin, a standard drug combination used in the treatment of leukaemia, exhibits drug ratio-dependent synergistic antitumor activity in vitro and in vivo. A cytarabine:daunorubicin molar ratio of 5:1 displayed the greatest degree of synergy and minimum antagonism in a panel of 15 tumor cell lines in vitro. Co-encapsulating cytarabine and daunorubicin inside liposomes maintained the synergistic drug ratio in plasma for 24h post-injection. Liposome-encapsulated cytarabine:daunorubicin combinations exhibited drug ratio-dependent in vivo efficacy with the 5:1 molar drug ratio (designated CPX-351) having the greatest therapeutic index, despite using sub-MTD daunorubicin doses. CPX-351 exhibited superior therapeutic activity compared to free-drug cocktails, with high proportions of long-term survivors, consistent with in vivo synergy. The therapeutic advantage of CPX-351 was associated with prolonged maintenance of synergistic drug ratios in bone marrow. These results indicate that in vitro informatics on cytarabine:daunorubicin cytotoxicity can be translated in vivo to optimize the efficacy of anticancer drug combinations by controlling the exposure of drug ratios with drug delivery vehicles.