Structure-based computational screening of the National Cancer Institute database of anticancer compounds identified novel non-nucleobase-derived inhibitors of human thymidine phosphorylase as candidates for lead optimization. The hierarchical in silico screening strategy predicted potentially strong low molecular weight ligands exhibiting a range of molecular scaffolds. Of the thirteen ligands assayed for activity, all displayed inhibitory activity against Escherichia coli thymidine phosphorylase. One compound, hydrazine carboxamide 2-[(1-methyl-2,5-dioxo-4-pentyl-4-imidazolidinyl)methylene], was found to inhibit E. coli thymidine phosphorylase with an IC(50) value of 20 microM and an IC(50) value of 77 microM against human thymidine phosphorylase. As this hydantoin derivative lacks the undesirable ionic sites of existing tight-binding nucleobase-derived inhibitors, such as 5-chloro-6-[(2-iminopyrrolidin-1-yl)methyl]uracil hydrochloride, it provides an opportunity for the design of potent thymidine phosphorylase inhibitors with improved pharmacokinetic properties.