Carboxylated poly(amidoamine) PAMAM dendrimers (generation 4.5) have been utilized for the synthesis and stabilization of ferrimagnetic iron oxide nanoparticles. Oxidation of Fe(II) at slightly elevated pH and temperature results in the formation of highly soluble nanocomposites of iron oxides and dendrimer, which are stable under a wide range of temperatures and pH's. Size exclusion chromatography indicates aggregates in the 20-30-nm size regime, consistent with the oligomeric nature of the composite material. Transmission electron microscopy reveals small assemblies of mineral cores with electron diffraction and high-resolution TEM data, indicative of the mineral maghemite. SQUID magnetometry demonstrated that this crystalline composition exhibited superparamagnetism at room temperature. NMR relaxation studies of solvent (water) protons revealed unusually high T1 and T2 relaxivities, which make these materials excellent candidates as contrast agents for MR imaging.