Water soluble Gd-based metallofullerenes have potential application as magnetic resonance imaging contrast agent due to its higher proton relaxivities and lower toxicity. In this Letter, we have investigated the structural and electronic properties of Gd@C-60, which is the most abundantly produced Gd-based metallofullerene in carbon arc process, by using all-electron relativistic density functional theory. It is found that the Gd3+ ion is bonded to a hexagonal ring of C-60 by electrostatic interaction. The total spin multiplicity of Gd@C-60 is S = 7. The Gd atomic orbitals are hybridized with the C-60 molecular orbitals. (c) 2006 Elsevier B.V. All rights reserved.