[en] We report self-catalyzed growth of GaAs nanowires (NWs) on Si/SiO_x patterns fabricated by a lithography-free method. The patterns are defined using droplet epitaxy of GaAs nanocrystals, spontaneous oxidation, and thermal annealing. We investigate the influence of the size and density of the nucleation sites on the NW growth process and show that this approach enables the fabrication of highly uniform GaAs NWs with controllable density. The pattern fabrication and NW growth process are studied and discussed in relation to the surface morphology and chemical properties of the Si/SiO_x patterns. Furthermore, the optical quality of the NWs is investigated by photoluminescence experiments performed for GaAs–AlGaAs core–shell NWs. (paper)