Nearly all water supply systems for high-rise buildings in Egypt include storage cisterns to feed the upper floors with cold water for different uses. Although these cisterns are the most important component in the distribution system, they are generally the least understood in terms of their effect on water quality. In this study, chlorine residual, algae genus and concentration, and pH value were measured before and after storage cisterns that exist above a 12-floor building in Assiut City, Egypt, to check their effect on water potability, through 48 h of observation. EPANET quality model is applied to simulate the chlorine residual and water age in the distribution system and storage cisterns through extended period simulations. The model is used to study the influence of the storage cisterns size and the turnover depth on residual chlorine decay and water age in the tanks. Results indicated that oversized storage cisterns can have negative impacts on water quality, including increased water age, reduced disinfectant residuals, and increased growth of disinfectant by-products (DBPs). However, there are other key roles that storage plays in the distribution system, e.g., security of supply during supply interruption and reserve storage for firefighting. Some of the microbiological and chemical effects of the cisterns on water quality have been investigated. DOI: 10 .1061/(ASCE)WR.1943-5452.0000132. © 2011 American Society of Civil Engineers.