Case studies from three Wisconsin utilities found that polyphosphate blends used for corrosion control led to increased metal concentrations in the water. When polyphosphate blends are added to water systems under certain conditions, leaching of pipe metal into the water may be significantly increased rather than decreased. Case studies of three Wisconsin water utilities uncovered possible negative consequences of using polyphosphate for corrosion control. These experiences suggest that water providers must exercise caution when adding polyphosphate to drinking water systems. Utilities should conduct off‐line tests before using polyphosphate, and full‐scale systems should be frequently monitored after polyphosphate addition. More research is needed to define the proper use of polyphosphates and to investigate alternatives to polyphosphates.