In order to identify the structural requirement at 4-position of 1-arylsulfonyl-4-phenyl-4,5-dihydro-2-limidazolones 3 for their cytotoxicity, the corresponding 1-arylsulfonyl-4-cyclohexyl-4,5-dihydro-2-imidazolones 4 were synthesized and their in vitro cytotoxicity against human solid tumor cell lines were measured. Unlike compounds 3a-c, cyclohexyl analogues 4a-c do not show the cytotoxicity. This dramatic loss of activity of these analogues on the volume change with the bulkier cyclohexyl group indicates that the planar structure at 4-position of 1-arylsulfonyl-4-pheny-4,5-dihydro-2-limidazolones 3 is required for their activity as an important pharmacophoric moiety.