The impact of climatic warming on lakes will most likely have serious implications for water resources and water quality. Rather than using model predictions of greenhouse warming, this paper looks at the changes in heat balance and temperature profiles in a particularly warm year (1988) compared to a more normal one (1971). The comparisons are made for three different morphometrically different lakes located 45°N latitude and 93°W longitude (north central United States) and for the summer period (April 1 to October 31). Water temperatures are daily values simulated with a model driven by daily weather parameters and verified against several sets of measurements. The results show that in the warmer year epilimnetic water temperatures were higher, evaporative water loss increased, and summer stratification occurred earlier in the season.