Swales are recognized as traditional basic open-drainage systems which are obviously capable to remove stormwater-borne pollutants. In spite of the numerous case studies on their performances, an overall assessment of swale efficiency to reduce pollutant discharge as well as an evaluation of the factors that could influence it are rare. In order to gain a better understanding of these aspects, a database was designed by collecting performance results and design characteristics from various swales reported in the literature. Investigations on correlations amongst pollutant efficiency ratios (ERs) indicated that total trace metals (copper (Cu), zinc (Zn), cadmium (Cd) and lead (Pb)), total suspended solids (TSS), total phosphorus (TP) and total kjeldahl nitrogen (TKN) exhibited many cross-correlated ERs, thus suggesting that they are mainly removed by the same processes in swales. Higher ERs were generally related to pollutants in particulate form, suggested that sedimentation and filtration should be considered as major processes involved in pollutant removal within swales. Furthermore, major discrepancies amongst swale performances may be attributed to variations of site mean inflow concentrations, found as the most recurrent factor influencing ER. Conversely, swale design factors were rather poorly correlated with ER, except swale length, positively correlated with TP and copper.