Abstract. Rainfall-derived infiltration and inflow (RDII) can interfere with the performance of domestic wastewater drainage systems. It is also a major cause for the deterioration of the functional performance in those systems and for the occurrence of domestic wastewater untreated discharges to the water environment. In most cases, the actual size and location of these inflows are unknown. To assess this subject of RDII, a detailed knowledge of the network is required as well as a diagnosis of the problem, namely, the type of inflows, the magnitude of their occurrence and the location of the most relevant impacts. This paper presents the application of a methodology to estimate RDII on a Portuguese case study.