This study assessed the water quality in Lake Wydmińskie (Warmińsko-Mazurskie Voivodeship) based on the content of both organic and inorganic pollutants flowing into the lake with water from drainage systems. The study demonstrated that the water flowing into the lake through drainage ditches that drain the improved part of the catchment area of the lake, was a potential source of diffuse pollutants and excess chemical components were deposited in the lake which could lead to its degradation. The most polluted water was found in the main tributary and the lake bay fed by its water. For this reason, it was concluded that the greatest problem was the area-wide sources of pollutants outgoing from agricultural areas. It was also found that water flowing out of the lake was of a considerably better quality than water flowing into the lake through the ditches. This clearly indicates the accumulative role of the lake on the route of pollutant migration from agricultural sources.