Surveys of lake water quality in northern Minnesota and Wisconsin reveal that approximately three-fourths of the lakes sampled are susceptible or potentially susceptible to acidification using the calcite saturation index as an approximate classification scheme. For some of the lakes in northern Wisconsin that fall above the upper curve, the threshold for acidification has already been exceeded. Limited data are available to determine the significance of the present atmospheric loadings of acids. Acid loadings by precipitation can be directly related to the resulting effects on lake pH as a dose-response relationship. Verification of these relationships and their application to individual watersheds is one of the most important research needs. If comparable, the results pertaining to ecological responses observed in Scandinavia and elsewhere can be directly translated to predicting, understanding and controlling the developing ecological effects being observed in North America.