The invention relates to a process for preparing chiral imidazolidin-2-ones of the formula I in which R1 is C1-C8-alkyl, cyclohexyl, phenyl, a C1-C6-alkyl-, halo-, nitro-, C1-C6-alkoxy-, C1-C6-alkylmercapto- or CF3-substituted phenyl radical, naphthyl or a C1-C6-alkyl-, halo-, nitro-, C1-C6-alkoxy- or CF3-substituted naphthyl radical, R2 is C1-C8-alkyl, C2-C8-alkenyl, cyclohexyl, phenyl or a phenyl-C1-C6-alkyl radical which may be substituted by a nitro, C1-C6-alkoxy, methylenedioxy or CF3 radical, and R3 is C1-C12-alkyl, C2-C8-alkenyl, cyclohexyl, phenyl or a C1-C6-alkyl-, halo-, nitro-, C1-C6-alkoxy-, methylenedioxy-, dialkylamino- or CF3-substituted phenyl radical, by reacting a compound of the formula II or the salt thereof in which R1, R2 and R3 have the abovementioned meaning, with urea in the presence of an involatile ammonium salt, wherein the reaction is carried out in the presence of an aprotic polar organic solvent.