This article describes the effectiveness of water utility quality control practices. There are three premises underlying this article. The first premise is that the data describe the effectiveness of water utility quality control in the broad sense that both the selection and surveillance of the source as well as treatment are a part of quality control. It is the quality of the end product, finished water, which shows how effective the overall process has been. The second premise is that the effectiveness of quality control is assessed by comparing the quality produced with standards. This is done categorically, and the standards chosen are the current U.S. Public Health Service Drinking Water Standards. The third premise is that the effectiveness of quality control is assessed from a national viewpoint, as the data come from 139 cities served by 194 sources or treatment plants. These are distributed uniformly across the nation, and plants in Alaska, Hawaii, and Puerto Rico are also included. As the selection was essentially random, it may be shown that the sampling is large enough to be statistically significant. Of these stations, over 67 per cent are reported here. The types of supplies included: 147 surface water; 39 groundwater; and, 8 groundwater and surface water.