A series of 3-(phenylsulfonyl)-1-phenylimidazolidine-2,4-dione derivatives have been synthesized and evaluated for their ability to selectively inhibit human heart chymase. The structure-activity relationship studies on these compounds gave the following results. The 1-phenyl moiety participates in a hydrophobic interaction where an optimum size is required. At this position, 3,4-dimethylphenyl is the best moiety for inhibiting chymase and showed high selectivity compared with chymotrypsin and cathepsin G. A 3-phenylsulfonyl moiety substituted with hydrogen-bond acceptors such as nitrile and methoxycarbonyl enhances its activity. Molecular-modeling studies on the interaction of 3-[(4-chlorophenyl)sulfonyl]-1-(4-chlorophenyl)-imidazolidine-2,4-dione (29) with the active site of human heart chymase suggested that the 1-phenyl moiety interacts with the hydrophobic P1 pocket, the 3-phenylsulfonyl moiety resides in the S1'-S2' subsites, and the 4-carbonyl of the imidazolidine ring and sulfonyl group interact with the oxyanion hole and the His-45 side chain of chymase, respectively. The complex model is consistent with the structure-activity relationships.