The pyrimidine analogues, used as antineoplastic agents, are a diverse group of agents with similar structures but somewhat different mechanisms of action, activities and spectra of activity. These agents are nucleoside analogues and are considered antimetabolites, interfering or competing with nucleoside triphosphates in the synthesis of DNA or RNA or both. The agents are analogues of cytosine (azacitidine, decitabine, cytarabine, gemcitabine) or uracil (fluorouracil, floxuridine, capecitabine), and demonstrate a range of antineoplastic activity in cell and animal models. Azacitidine and decitabine have unique actions in that they block or decrease the methylation of cytosine and, thus, cause hypomethylation of DNA and increased gene expression. Because some cancers are marked by hypermethylation of tumor suppressor genes, azacitidine and decitabine have a potential for specific anticancer activity in these conditions. These two agents are used predominantly in the therapy of myelodysplasia. In contrast, fluorouracil (5-FU) and floxuridine (FUDR) have more typical antineoplastic activity and are important agents in regimens for several solid tumors. Capecitabine is an orally available, prodrug of fluorouracil and has activity against a similar spectrum of cancers as fluorouracil. Cytarabine (Ara-C) and gemcitabine are cytosine analogues, but are used in different forms of cancer, cytarabine for leukemias and lymphomas and gemcitabine in solid tumor chemotherapy.