Lake Baldegg, in the center of Switzerland, has an area of 5.2 km2 and a maximum depth of 66 m. First due to untreated sewage of the settlements and food industries, later due to more intense agriculture and higher animal densities, phosphorous concentration in the lake increased from 80 to 500 mg P/m3 during the period from 1955 to 1975. In the year 1980 different steps were taken by the government of Luceme to reduce the external input of phosphorous into Lake Baldegg. Based on long-term investigations, the reaction of Lake Baldegg on internal and external measures is described in this article. The deep-water zone became aerobic again, and reduced substances as NH4+, H2S and CH4 disappeared. The living space for fish has been extended into deeper layers. By means of stocking of white fish breedlings, the white fish yield could be increased. Worms and chironomids were found again down to the deeper sediments. The phosphorous concentration in Lake Baldegg decreased in the meantime to 80 mg P/m3, mainly caused by external measures as sewage treatment with phosphorous removal. Based on long-term experiments, new goals for lake sanitation are postulated. The question will be answered, what would happen if the artificial oxigenation would be stopped abruptly.