Significance Our work shows a remarkable synergy between physiological levels of vitamin C and 5-aza-CdR. The combination enhances the viral mimicry response to DNA methyltransferase inhibitors, including the upregulation of endogenous retroviruses in the dsRNA form and the induction of viral defense pathways. Because patients with hematological and other cancers are often markedly vitamin C deficient, the addition of vitamin C to treatment protocols may be a straightforward way to increase the clinical efficacy of such drugs in patients with myelodysplastic syndrome and leukemia.