Eighty-four percent of Ohio's lakes and reservoirs are eutrophic. Significant reductions in nutrient loading are usually the object of lake restoration projects. However, variations in attainable nutrient concentrations, based on regional differences in watershed characteristics and land use, often are not considered. The purpose of this project was to evaluate the “ecoregion” concept for estimating which surveyed Ohio reservoirs might have the greatest potential for reduction in phosphorus concentrations. Ecoregional watershed characteristics dictate the practical lower or attainable limits of stream nutrient concentrations that can be reached. Attainable reservoir phosphorus concentrations were estimated from stream phosphorus concentration data that represent the best water quality in each Ohio ecoregion and by the use of a loading model. The reservoirs with actual summer 1989 phosphorus concentrations that exceeded predicted attainable concentrations by the greatest amounts were considered to have the greatest potential for restoration. These were not reservoirs with the worst trophic states. Similar findings have been reported for Minnesota (Heiskary et al. 1987). The method proposed here could be used statewide to establish water quality standards, provide a rational way to establish a priority program for lake restoration, and develop lake improvement and protection goals.