The average length of in-patient stay (LOS) in Japan is longer than that of the other Organization for Economic Cooperation and Development (OECD) countries. The tendency towards long LOS is also apparent in teaching hospitals in Japan. This paper examines factors responsible for the long LOS in teaching hospitals, focusing on conditions of residency training. The study was conducted as a part of the first nation-wide study of teaching hospitals for postgraduate medical education in Japan and covered most teaching hospitals (61 university hospitals out of 80 and 153 non-university general hospitals out of 193). The multivariate analysis suggested that longer LOS was more common in the hospitals with lower autopsy rate per bed, higher operation rate per admission, and smaller number of in-patients (P