Results of initial operation of a computer-based data acquisition and control system for a major urban combined sewer system are presented. Impressive reductions in combined sewer overflow pollution of the Mississippi River in Minneapolis and St. Paul have been effected. Working entirely within the limits of the existing interceptor sewer system, and with relatively minor modifications to selected major combined sewer regulators, incidence of overflow was reduced by 66% and duration of overflow by 88% during most of a rainfall season. Computer simulation techniques using actual rainfall data indicate that the amount of overflow volume reduction achieved is the equivalent of a $200 million separation project. A mathematical model has been developed that will, with rain gage data as input, perform runoff analysis, diversion of combined sewer runoff hydrographs at regulators, and routing of diverted hydrographs through the interceptor system. (Author)