Three different sizes of Eu0.2Gd0.8PO4 center dot H2O nanoparticles have been prepared to investigate the particle size influence on water proton relaxivity. Longitudinal relaxivity (r(1)) values increase for smaller particles, reaching as high as r(1) = 6.13 mM(-1) s(-1) for a sample of 40 +/- 4 nm particles, which, with a ratio of transverse/longitudinal relaxivity, r(2)/r(1) = 1.27, are shown to be effective positive contrast agents. The correlation between relaxivity and the surface-to-volume ratio implies that access to surface Gd3+ sites is the principal factor affecting relaxivity. On the other hand, although ionic molar relaxivity decreases for larger particles, the relaxivity per particle can be significantly greater. Gadolinium-based nanoparticles doped with fluorescent lanthanide elements have attracted attention for their dual-imaging abilities, combining magnetic resonance imaging (MRI) and fluorescence imaging agents. In both in vitro experiments with He La cells and in vivo experiments with C. elegans, strong red fluorescence is observed from Eu0.2Gd0.8PO4 center dot H2O with high resolution, demonstrating the parallel use of the particles as fluorescence imaging agents.