The transverse H-1 relaxivities of aqueous colloidal solutions of dextran coated DY2O3 nanoparticles of different sizes were investigated at magnetic field strengths (B) between 7 and 17.6 T. The particle size with the maximum relaxivity (r(2)) appears to vary between 70 nm at 7 T (r(2) approximate to 190 s(-1) mM(-1)) and 60 nm at 17.6 T (r(2) approximate to 675 s(-1) mM(-1)). A small difference between r(2) and r(2)(*) was observed, which was ascribed to the effect of the dextran coating. The value of r(2) is proportional to B-2 up to 12 T after which it saturates. Independent magnetization measurements on these particles at room temperature at magnetic field strengths up to 30 T, however, show a typical paramagnetic behavior with a magnetization of the particle that is proportional to the field strength. The saturation in the curve of r(2) as a function of B-2 was tentatively explained by the presence of an extremely fast relaxing component of the signal at high field strengths, which is not observable on the NMR time scale. The results of this study can be exploited for the rational design of MRI contrast agents, based on lanthanicle oxide particles, with high efficiencies at magnetic field strengths of more than 1.5 T.