The relationship between electronic and atomic structure of quantum confined objects is a key issue in nanoscience. We performed low-energy electron diffraction to determine the surface relaxations of ultrathin Pb films on Si1117 7 with thicknesses ranging from four to nine atomic layers. The results indicate a contraction of the first interlayer spacing d12 for all films. The d12 contraction exhibits a small bilayer modulation as a function of thickness, indicative of a quantum size effect. The oscillatory relaxations furthermore suggest an interesting correlation with the work function oscillations predicted from density functional theory Wei and Chou, Phys. Rev. B 66, 233408 2002 and can be understood qualitatively on the basis of the jellium model.