We previously designed 1-(3-C-ethynyl-beta-D-ribo-pentofuranosyl)uracil (EUrd) as a potential multifunctional antitumor nucleoside antimetabolite. It showed a potent and broad spectrum of antitumor activity against various human tumor cells in vitro and in vivo. To determine the structure-activity relationship, various nucleobase analogues of EUrd, such as 5-fluorouracil, thymine, cytosine, 5-fluorocytosine, adenine, and guanine derivatives, were synthesized by condensation of 1-O-acetyl-2,3,5-tri-O-benzoyl-3-C-ethynyl-alpha,beta-D-ribo-pentofur anose (6) and the corresponding pertrimethylsilylated nucleobases in the presence of SnCl4 or TMSOTf as a Lewis acid in CH3CN followed by debenzoylation. The in vitro tumor cell growth inhibitory activity of these 3'-C-ethynyl nucleosides against mouse leukemia L1210 and human nasopharyngeal KB cells showed that 1-(3-C-ethynyl-beta-D-ribo-pentofuranosyl)cytosine (ECyd) and EUrd were the most potent inhibitors in the series, with IC50 values for L1210 cells of 0.016 and 0.13 microM and for KB cells of 0.028 and 0.029 microM, respectively. 5-Fluorocytosine, 5-fluorouracil, and adenine nucleosides showed much lower activity, with IC50 values of 0.4-2.5 microM, while thymine and guanine nucleosides did not exhibit any activity up to 300 microM. We next evaluated the tumor cell growth inhibitory activity of ECyd and EUrd against 36 human tumor cell lines in vitro and found that they were highly effective against these cell lines with IC50 values in the nanomolar to micromolar range. These nucleosides have a similar inhibitory spectrum. The in vivo antitumor activities of ECyd and EUrd were compared to that of 5-fluorouracil against 11 human tumor xenografts including three stomach, three colon, two pancreas, one renal, one breast, and one bile duct cancers. ECyd and EUrd showed a potent tumor inhibition ratio (73-92% inhibition relative to the control) in 9 of 11 and 8 of 11 human tumors, respectively, when administered intravenously for 10 consecutive days at doses of 0.25 and 2.0 mg/kg, respectively, while 5-fluorouracil showed potent inhibitory activity against only one tumor. Such excellent antitumor activity suggests that ECyd and EUrd are worth evaluating further for use in the treatment of human cancers.