The aim of this paper is to assess the performance level of water distribution systems (WDS) in six areas located in four Mediterranean countries, and evaluate their common characteristics. The second modification of the IWA Standard International Water Balance was used to account for the fixed charge included in the customers' bills. Each WDS efficiency was annually assessed, although in particular cases, shorter time periods (6 and 4 months) were applied depending upon local conditions, data availability, etc. The main common characteristic found was that some data needed to perform water balance calculations were missing (e.g. regarding the level of unbilled authorized use, unauthorized use, and customer meter inaccuracies). Moreover, a fixed-charge billing policy was adopted by the local utilities in all six cases, underestimating the impact of the high non-revenue water (NRW) values. This policy results in misleading, low values of unrecoverable revenues, although water losses remain high. Thus, water utilities are not motivated to develop NRW reduction strategies. Nevertheless, the analysis revealed that real losses represent the main NRW component in all case studies, thus respective reduction strategies should focus on them. Wherever WDS monitoring, simulation and leakage reduction measures were established, lower NRW values occur.