Based on data that none of the 53 registrated and 60 000 illegal landfill sites in Slovenia is properly structured, the landfill leachate is deemed to be one of the most serious pollution problems. A pilot constructed wetland (CW), as an innovative technology that provides simple and inexpensive waste water treatment was introduced for landfill leachate treatment at the Dragonja landfill site on the Adriatic coast. The system consists of two interconnected beds with a subsurface flow, covering an area of 450 m2. An average hydraulic load was 3 cm d−1. The influent concentrations were for COD 1264 mg l−1, BOD5 60 mg l−1, NH3-N 88 mg l−1, undissolved solids 400 mg l−1, and Fe 10 mg l−1. Monitoring of CW performance was initiated in 1992 by analysing physical, chemical and microbiological parameters. Evident reductions after the initial period have been achieved in COD (68%), BOD5 (46%), NH3-N (81%), Fe (80%), and bacteria (85%). The results show that CW was fairly efficient, although it was very difficult to determine the proper area especially because of specific hydraulic and pollution fluctuations. Therefore, further investigations should give more data on proper design.