As the illumination of buildings at night increases, light pollution and negative impacts on wildlife also increase. In order to assess the effect of direct lighting on house-dwelling bats, we examined colonies of Rhinolophus ferrumequinum, Myotis emarginatus and M. oxygnathus in illuminated and non-illuminated buildings found in close proximity to each other. We investigated the onset and timing of nocturnal emergence and measured the body mass and the forearm length of juvenile bats. Results show that bright artificial lighting delays the onset or significantly prolongs the duration of emergence and, in the worst cases, may destroy the whole colony. Juveniles are significantly smaller in illuminated buildings than in non-illuminated ones. The differences in length of the forearm and in body mass may suggest that the parturition time starts later and/or the growth rate is lower in bats living in illuminated buildings. Thus, the illumination of buildings could have serious implications for the conservation of house-dwelling bat colonies.