1-p-D-Arabinofuranosylcytosine (ara-C) is one of the most potent drugs for the treatment of acute human leukemia, but it has drawbacks including a very short half-life due to deamination to chemotherapeutically inactive 1-p-D-arabinofuranosyluracil by cytidine deaminase and ineffectiveness against solid tumors. Efforts have been made to develop prodrugs or introduce substituents into the 2'-arabino position of 2'-deoxycytidine to overcome these issues, with some analogs (e.g., 2'-azido-, 2'-amino-, (2'S)-2'-methyl-) showing activity. Considering the effects of substituent bulkiness, polarity, and electronegativity on sugar conformation and enzyme recognition, and the structural features of nucleoside antibiotics (e.g., angustmycin A, neplanocin A) with allylic alcohol systems (which may enhance reactivity or fix sugar conformation), we synthesized 2'-deoxy-2'-methylidenecytidine (DMDC) from uridine in eight steps. We evaluated its antineoplastic activity in vitro using mouse and human tumor cell lines. DMDC exhibited broad inhibitory activity, acting at low concentrations against not only mouse leukemic (L1210) and human leukemic (CCRFCEM, MOLT 4, K562) cell lines but also human carcinoma (PC1, SW480, TE2, T24) cell lines. Notably, DMDC was resistant to cytidine deaminase (no significant deamination in 2 h, whereas cytidine and ara-C were deaminated 100% and 88%, respectively). Like ara-C, DMDC specifically inhibited DNA synthesis (98% inhibition of [³H]thymidine incorporation into DNA at 10 µg/mL) without affecting RNA or protein synthesis in L1210 cells.