Seasonal and multiannual hydrological changes were analysed in seven lakes of northeastern Poland on the basis of a 40-year series of everyday observations of the water level in the years 1961-2000. In the unmodified hydrological lakes occurs an unimodal cycle of variation in the water level with a spring maximum, whose magnitude depends on atmospheric precipitation in the form of snow in the period December- March. In lakes with hydrology transformed by man, seasonal water level changes depend on their economic function in the catchment. An anthropogenic increase in the amplitude of the varying water level in lakes is the factor intensifying the eutrophication of water bodies. Multiannual cycles of variation in lake levels take different spans of time depending on hydrological characters of the lakes.