Purine nucleoside phosphorylase (PNP, EC 2.4.2.1) is a salvage enzyme important to the T-cell-mediated part of the immune system and as such is an important therapeutic target. This paper describes the design, synthesis, and enzymatic evaluation of potent, competitive inhibitors of PNP. Potential inhibitors were designed using the three-dimensional structure of the enzyme in an iterative process that involved interactive computer graphics to model the native enzyme and complexes of it with the inhibitors, Monte Carlo-based conformational searching, and energy minimization. Studies of the enzyme/inhibitor complexes were used to determine priorities of the synthetic efforts. The resulting compounds were then evaluated by determination of their IC50 values and by X-ray diffraction analysis using difference Fourier maps. In this manner, we have developed a series of 9-(arylmethyl)-9-deazapurines (2-amino-7-(arylmethyl)-4H-pyrrolo[3,2-d]-pyrimidin-4-ones) that are potent, membrane-permeable inhibitors of the enzyme. The IC50 values of these compounds range from 17 to 270 nM (in 1 mM phosphate), with 9-(3,4-dichlorobenzyl)-9-deazaguanine being the most potent inhibitor. X-ray analysis explained the role of the aryl groups and revealed the rearrangement of hydrogen bonds in the binding of the 9-deazaguanines in the active site of PNP relative to the binding of the 8-aminoguanines that results in more potent inhibition of the enzyme.