A new class of potential magnetic resonance imaging contrast agents with nanosized mesoporous silica as the metal carrier is reported. Gadolinium-incorporated mesoporous silicas were synthesized by using longchain surfactant as a template. The products were characterized with X-ray powder diffraction, nitrogen adsorption-desorption isotherms, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, Si-29-MAS NMR, and proton relaxivity. The materials showed much higher relaxivities, r(1) and r(2), than Gd-DTPA. The particles are nanosized and can enter cells easily. This is a completely novel biomedical application of mesoporous silica materials.