In this communication, we demonstrate that there is an optimum gadolinium oxide (Gd(2)O(3)) nanoparticle size of 2.3 nm; in the presence of Gd(2)O(3) particles smaller and larger than this critical size, the spin-lattice relaxation rate (T(1) = 1/r(1)) of water protons at 7.0 T drastically decreases. Since r(1) is directly related to the quality of magnetic resonance imaging, the results presented here have significant implications for clinical diagnostics.