Starting with 5-iodo-2'-deoxyuridine, a series of 5-alkynyl-2'-deoxyuridines (with n-propyl, cyclopropyl, 1-hydroxycyclohexyl, p-tolyl, p-tert-butylphenyl, p-pentylphenyl, and trimethylsilyl alkyne substituents) have been synthesized via the palladium-catalyzed (Sonogashira) coupling reaction followed by a simplified isolation protocol (76-94% yield). The cytotoxic activity of modified nucleosides against MCF-7 and MDA-MB-231 human breast cancer cells has been determined in vitro. 5-Ethynyl-2'-deoxyuridine, the only nucleoside in the series containing a terminal acetylene, is the most potent inhibitor with IC(50) (microM) 0.4+/-0.3 for MCF-7 and 4.4+/-0.4 for MDA-MB-231.