The urbanization growth on a global scale has intensified conflicts over the use of water resources, not only because the increased demand, but also because of supply reduction, which is the result of an unplanned development that deteriorates water quality. Thus, in this work we assessed the impact of Piracicamirim Wastewater Treatment Plant (WTP), inaugurated in 1998, over the water quality of the Piracicaba River Hydrographical Basin, where urbanization in the 1960's was stimulated by industrial decentralization in the Metropolitan Region of São Paulo. Through the information provided by the State Environmental Company of São Paulo (CETESB) it was possible to explore the annual series of the variables dissolved oxygen, biochemical oxygen demand and fecal coliform, besides the Water Quality Index, for a period between 1980 and 2012. The results indicated that in the period before the implementation of the WTP there was a significant deterioration in the water quality of the Piracicaba River between the late 1980's and early 1990's. The WTP has prevented the deterioration to intensify in what seems to indicate a run between unbridled growth and the capacity of the sewage system. This work highlights the importance of planning and integration between sanitation and urbanization policies.