An experimental and theoretical study of magnetic circular dichroism in valence band photoemission from 15 monolayer thick fcc Ni films on Cu(001) is presented. A highly symmetric configuration (light incidence, electron emission, magnetization direction, photon helicity, and surface normal all parallel) allows the illustrative interpretation of the dichroism in terms of the relativistic band structure. Photoemission experiments in the photon energy range of 11-27 eV are compared to fully relativistic one-step photoemission calculations. From this comparison, the dichroic features can be directly related to the double group symmetry of the initial states, which is demonstrated by two examples.