The relationship between leak outflow from a damaged pipe and flow condition inside the pipe plays a crucial role in pressurized pipe systems management. As an example, this relationship is used in leakage reduction techniques based on pressure control and in leak detection techniques based on inverse analysis. To explore the relationship between total head inside the pipe and leak outflow for a single leak in a polyethylene pipe, tests were carried out at the Water Engineering Laboratory of the University of Perugia. These tests point out that the viscoelastic nature of the pipe material gives rise to a hysteretic behavior of the investigated relationship, i.e., the outflow depends not only on the synchronous total head but also on the total head time history and variation rate. DOI: 10.1061/(ASCE)HY.1943-7900.0000360. (C) 2011 American Society of Civil Engineers.