Complexes of YIII, LaIII, GdIII and DyIII with an 18-membered hexaaza tetraimine macrocyclic ligand (L1) containing backbone cyclohexyl units have been prepared and characterized. The crystal structure of [GdL1(H2O)3]Cl3·3H2O shows a nine-co-ordinate gadolinium ion bound to three water oxygens and six nitrogen donors from the macrocyclic ligand. Solution studies with radiolabelled 90Y indicated that, in competition with the very strong chelator diethylenetriamine-N,N,N′,N″,N″-pentaacetic acid (H5dtpa), a 90YIIIL1 macrocyclic unit is maintained for 1 h. The relaxivity of the gadolinium(III) complex, [GdL1(H2O)3]3+, is higher than those of [Gd(dtpa)(H2O)]2– and [GdL(H2O)]–(H4L = 1,4,7,10-tetraazacyclododecane-N,N′,N″,N[triple prime]-tetraacetic acid), both of which are currently used as contrast agents in magnetic resonance imaging. A tetramethyl analogue of L1, i.e. L2, has been synthesized for the first time and a crystal structure determination showed that it adopts a stepped conformation, the direction of folding being dictated by the racemic conformation of the cyclohexane.