From the historical perspective, the increasing number of inhabitants in the city of Stockholm has had a negative effect on water quality in the surrounding lakes and coastal region. Government control and measures taken to improve water quality have been in progress since the mid-19^th century. Water conditions in the 19^th century, compared to later years is difficult to assess from the first chemical and biological investigations, due to infrequent sampling and the different parameters and methods used. However, a retrospective evaluation of water quality can be made on the basis of results from plankton investigations, which began in the early 20^th century. The occurrence of the cyanobacteria Planktothrix agardhii, which indicates nutrient-rich conditions, was surveyed during summers with similar temperature conditions throughout the 20^th century. The results show that eutrophied conditions probably prevailed in the Stockholm archipelago from the beginning of 20^th century until the early 1990s. In the last decade of the 20^th century, water quality appeared to be better than 100 years earlier. Today's better water conditions are most probably an effect of proper measures taken in wastewater treatment.