Gadolinium hydroxide, Gd(OH)(3), nanostructures were examined for their possible use in imaging and tracking of cells and tissues by investigating their cellular interactions and cytotoxic behaviors. For this purpose, Gd(OH)(3) nanorods (length, several hundred nanometers; diameter, similar to 40 nm) and spherical nanoparticles (average diameter, <10 nm) were synthesized by solvothermal decomposition of gadolinium containing molecular precursors. After comprehensive characterization of material properties, human colon adenocarcinoma (Caco2) and human lung epithelial (A549) cells were incubated with Gd(OH)(3) nanostructures in concentrations up to 900 mu g/mL to perform cytotoxicity assays (BrdU (5-bromo-2'-deoxyuridine), WST-1 (4-[3-(4-iodophenyl)-2-(4-nitrophenyl)-2H-5-tetrazolio]-1,3-benzol-disul fonate)) and living/dead staining. As observed in all in vitro assays, the investigated Gd(OH)(3) nanostructures do not induce any significant cytotoxic effect, making them promising candidates for a new class of contrast agents, which may overcome the limitations of organic stains such as photobleaching and single usage.