We have investigated the homoepitaxial growth of Ag(100) at room temperature by means of surface X-ray diffraction. The out-of-phase specular reflection shows intensity oscillations during growth indicating layer-by-layer growth. From transverse in-plane profile scans the island-size distribution is determined at various stages during the growth. Pre-deposition of Sb on the clean surface improves the layer-by-layer growth by enhancing the interlayer transport. The lineshape of the peak profiles indicates that the preferred terrace size has disappeared in this case, showing that the randomly deposited Sb atoms act as nucleation centers for the formation of Ag islands.