Objective This study aimed to evaluate the effects of establishing a Department of General Internal Medicine (DGIM) on the length of hospitalization. We evaluated the length of hospitalization associated with diseases for which full-time specialists were not available and were instead treated by physicians of the DGIM after its establishment. Methods A retrospective cohort study was conducted with a review of the subjects' medical records. The subjects included patients ! 16 years of age who were hospitalized with pneumonia or cerebral infarction and treated by a physician with a specialty in internal medicine as the disease outside their specialty prior to DGIM establishment (October 1, 2006 to September 30, 2008 or by a physician of the DGIM after its establishment (October 1, 2009 to September 30, 2011). The primary outcome was the change in the length of hospitalization. The length of hospitalization for heart failure, which was treated by specialists (cardiologists) in both study periods, was also examined for comparison. Results We evaluated 322 and 423 cases of pneumonia treated before and after the establishment of the DGIM, as well as 223 and 229 cases of cerebral infarction and 132 and 206 cases of heart failure, respectively. The length of hospitalization before and after establishment of the DGIM was 21.6 and 16.0 days for the pneumonia patients (p<0.001) and 24.2 and 19.9 days for the cerebral infarction patients (p<0.001), respectively. On the other hand, the change in the length of hospitalization for the heart failure patients was not statistically significant (19.9 vs. 17.6 days; p=0.281). Conclusion The establishment of the DGIM reduced the length of hospitalization associated with diseases for which full-time specialists were not available by over 17%.