To evaluate the role of imidazolidinone moiety of potential anticancer 4-phenyl-1-arylsulfonylimidazolidinones 1 for their cytotoxicity conformationally similar 4-phenyl-2-arylsulfonylaminooxazolines 2 were synthesized and compared their cytotoxicities with those of the corresponding 1. Compounds 2 showed much reduced activity compared to N-arylsulfonylimidazolidinones 1. This result might indicate that the imidazolidinone ring of 1 have the other roles for the activity as an essential structural motif in addition to conformational contribution .