In 2003, a lump-sum payment system based on Diagnosis Procedure Combinations (DPC) was introduced to 82 specific function hospitals in Japan. While the US DRG/PPS system is a "per case payment" system, the DPC based payment system adopts a "per day payment." It is generally believed that the Japanese system provides as much of an incentive as the DRG/PPS system to shorten the average length of stay (LOS). We performed an empirical analysis of the effect of LOS shortening on hospital revenue and expenditure under the DPC-based payment system, particularly in cardiovascular diseases. We also point out fundamentally controversial aspects of the current system. A total 109 cases were selected from patients hospitalized at the University of Tokyo Hospital from May to July, 2003 and classified into one of three categories: (1) cardiac catheter interventions, (2) cardiac catheter examinations, and (3) other conservative treatments. We analyzed the changes in profit per day in cases of a reduction in average LOS and an increase in the number of cases. In category (1) profit increased significantly in conjunction with reduced LOS. In category (2) profit increased only minimally. In category (3), profit increased rarely and sometimes decreased. In cases of conservative treatment, profits sometimes decreased because an increase in material costs exceeded the increase in revenue. It therefore became clear that the DPC-based payment system does not decisively provide an economic incentive to reduce LOS in cardiovascular medicine.