The importance of a water supply and sewage treatment for urban sanitation is recognized in the modern world. Their contributions to public health have not, however, been well demonstrated by historical data, especially in Asian cities. In this research, we focused on the Asian cities of Tokyo and Singapore, which both developed significantly in the 20th century. We analysed their development processes statistically to determine what the key elements for the protection of urban sanitation have been. Although both cities constructed modern water supply systems at almost same time (Tokyo in 1898 and Singapore in 1878), and similarly modern wastewater treatment systems (Tokyo in 1922 and Singapore in 1913), the prevalence of water-borne diseases in Tokyo was more serious than it was in Singapore, in spite of Singapore's high infant mortality rate. The main reason for this was the differences in the systems of night-soil transport. We found that the water supply system in itself was not enough to resolve all urban sanitation problems, and appropriate night-soil removal was also crucial. In addition, historical trends and water consumption vary by city, so the appropriate technology and system are also different according to the unique characteristics and needs of each.