A lake evaluation index (LEI) is proposed to express the overall quality of lake water based on a mathematical combination of the commonly measured limnological variables total phosphorus, total nitrogen, chlorophyll, Secchi depth, dissolved oxygen, and macrophyte coverage. Values used in the LEI are averages for the July-August period. Macrophyte coverage is expressed as the percent of the area which could potentially be occupied actually. The LEI was developed to express the overall change in water quality that might occur when lakes are restored and to evaluate the effectiveness of various treatment methods for a wide variety of lakes.