The study is performed according to the operational monitoring guidance in the Norwegian “Vannforskriften”, which is the Norwegian implementation of EUs Water Framework Directive. The results show that Lake Harestuvannet has good/very good ecological status, and is little affected by the discharges from sewage works. The surface waters of the lake also satisfied the norms for bathing water quality. The oxygen concentration in the deep water during the summer stagnation was relatively low, and seemed lower than measured in previous studies, whereas the oxygen concentration in the deep water at end of the winter stagnation period was good. The deep water area of the lake is restricted, and the ecological impact of the possibly increased oxygen demand is small.