A new route to the synthesis of colloidal Au nanoparticles with diameters between 30 and 100 nm is described. On the basis of surface-catalyzed reduction of AU(3+) by NH2OH, the approach grows existing nanoparticles into larger particles of a size determined solely by the initial particle diameter and the amount of Au3+ added. The resulting particles exhibit improved monodispersity relative to those prepared in one step by the reduction of Au3+ by citrate. Importantly, surface-confined colloidal Au nanoparticles can also be enlarged by this method, providing an attractive route to colloidal Au monolayers with variable interparticle spacing (at fixed particle coverage).