New 3-deaza-3-halouracil nucleosides including 3-deaza-3-fluorouridine and its 2'-deoxy and arabino analogues have been prepared by fluorination of protected precursors. The resulting 3,3-difluoropyridine-2,4(1H,3H)-dione derivatives underwent palladium-catalyzed hydrogenolysis of one C-F bond at atmospheric pressure, and deprotection gave the 3-deaza-3-fluorouracil compounds. Selective reaction of a stabilized Wittig reagent at C4 of the 3,3-difluoro-2,4-dione intermediates gave exocyclic alkenes that underwent hydrogenation accompanied by spontaneous elimination of hydrogen fluoride. Ammonolysis of the exocyclic carbethoxymethyl substituent and ester protecting groups gave 4-(carboxamidomethyl)-3-deaza-3-fluorouridine and its analogues. Grignard additions at C4 of the ribo and 2'-deoxy 3,3-difluoro-2,4-dione intermediates followed by deprotection gave the 3-deaza-3,3-difluoro-4-hydroxy-4-(substituted)uracil nucleosides. The cytostatic activity of 3-fluoro-3-deazauridine (CC(50) = 4.4-9.6 microM) in three cancer cell lines paralleled that of 3-deazauridine, whereas no significant inhibitory activity was observed with a variety of virus-infected cell cultures.