Alum treatment of lakes has been found to be a generally effective method for reducing P concentrations in lakes over a period of 2 to 3 years after treatment. However, the long-term benefits of lake alum treatment in most cases have not been determined. In this study, three Wisconsin lakes treated 9 to 12 years previously were re-examined in 1982 to evaluate the long-term effects of the treatment projects. Results indicated that many years after treatment P levels in all three lakes remained lower than before the lakes were treated. Horseshoe Lake, the first U.S. lake treated with alum in 1970, was more eutrophic in 1982 than immediately after treatment but is still much improved over pretreatment conditions. The reduced levels of phosphorus in 1982 compared with before the alum treatment, indicate the alum layer is preventing the migration of phosphorus from the deep sediments. The 1982 phosphorus concentrations in the hypolimnion of Snake Lake are similar to levels experienced following the 1972 treatment despite continued input of stormwater runoff to the lake. While the previous two lakes are dimictic, Pickerel Lake is a polymictic lake. The increased mixing action has redistributed much of the alum toward the center of the lake. The alum treatment appeared to have little effect on the internal phosphorus dynamics of the lake.