We have used the temperature depth profiles available in the region north and northwest of Lake Superior, in Ontario, Canada, to reconstruct the changes in ground surface temperature over the past 500 years. The 49 temperature depth profiles used were obtained for heat flow measurements and are part of two different data sets, one collected around 1980, the other one after 2000. We have discarded 16 of these profiles because of known nonclimatic perturbations (lakes, topography, clear signs of groundwater circulation). We have inverted the remaining 33 profiles to infer the variations in ground surface temperature. Individual and joint inversions consistently show a recent (150--200 years) warming (1--2 K) of the ground surface. This warming trend is similar to that inferred for northern Manitoba and Saskatchewan, to the northwest, and in eastern Ontario and Qu¿bec, to the southeast. However, there is no clear indication that a cold episode preceded the warming of the past 200 years in northwestern Ontario. The mean increase in ground surface temperature for all boreholes measured between 2000 and 2003 is 1.3 ¿ 0.8 K compared to 0.8 ¿ 0.6 K (σ) for the 1980 data. This difference suggests that the warming trend is persisting and might even have accelerated recently. Although the long-term trend is climatic, part of the recent warming may be due to deforestation by the logging industry.