A new convenient and easy-scalable one-step synthetic strategy to achieve metal-containing polymer nanoparticles for applications as magnetic resonance imaging contrast agent is reported. In this study, a novel contrast agent based on poly(gadolinium methacrylate) (poly(Gd(MAA)(3))) nanoparticles was prepared by one-step aqueous dispersion polymerization of gadolinium methacrylate monomer (Gd(MAA)(3)), whereby stable particles were obtained due to the association of Gd-III with the polymer carboxylate anions, which provided latent crosslinking of the polymer nanoparticles without the addition of further crosslinking reagents. The morphology and final composition of the corresponding nanoparticles was thoroughly characterized and their cytotoxicity as well as their potential use in MRI was evaluated in vitro on HEK 293T cells by using the CCK-8 assay. The presented results demonstrated, that the poly(Gd(MAA)(3)) nanoparticles had a spherical morphology with mesoporous substructure, a sufficiently low cytotoxicity and an exceptionally high longitudinal relaxivity of r (1) = 12.613 mM(-1) s(-1), making these nanoparticles excellent candidates for in vivo imaging systems. Herein described poly(Gd(MAA)(3)) nanoparticles can be valuable in a wide range of biomedical applications with simultaneous bioconjugation, drug delivery as well as imaging capabilities for the early detection of lesions of the brain and the central nervous system, for assessing cardiac function, and for detecting tumors.