Abstract We investigate the luminosity evolution of the host galaxies of radio-loud AGN through Hubble Space Telescope imaging of 72 BL Lac objects. This sample has enabled us to avoid the biases implicit in quasar studies. We find that the host galaxies of BL Lacs evolve strongly, consistent with that observed in the hosts of other radio-loud AGN, and inconsistent with the flatter luminosity evolution of quiescent early types and radio-quiet hosts. This indicates that active star formation, and hence galaxy interactions, are associated with the formation of radio-loud central engines, and that these host galaxies preferentially accrete less material after their formation epoch than galaxies without powerful radio jets.