In this work, size-controlled multifunctional mesoporous silica nanoparticles having large surface areas, embedded luminescence, high magnetization, and excellent aqueous dispersity have been successfully prepared by using a simple one-pot synthesis. The size and pore ordering of these particles can be easily controlled based on the number density of Fe 3O 4 nanoparticle nucleation sites introduced during the silica condensation reaction. Dissolution of the embedded Fe 3O 4 nanoparticles yields hollow mesoporous silica nanoparticles as well. These multifunctional porous nanoparticles were characterized by transmission electron microscopy, X-ray diffraction, nitrogen adsorption-desorption behavior, dynamic light scattering, zeta potential, magnetic susceptibility, and photoluminescence. Furthermore, in vitro 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-tetrazolium bromide (MTT) and hemolysis assays were performed to evaluate any unintended cytotoxicity. The biocompatibility of the multifunctional nanoparticles, even at very high doses, ensures their potential in biomedical applications.