The character of continental-interior paleoclimate at mid-latitudes, especially the aspect of temperature, has been a major source of debate in modeling and paleontologic communities over the past few years. A recent climate modeling study provides new insight into the issue of climatic conditions of early Eocene North America. Model cases with six times the present level of CO2, or with doubled present CO2 and a large lake in western North America, produced results most similar to proxy paleoclimate interpretations. It is significant that the North American continental-interior climate responded as strongly to the existence of the lake as to the atmospheric CO2 level. The large lake deflects the winter-freeze line poleward of the region containing most paleoclimate data site locations, producing above-freezing winter temperatures and providing a possible solution to the minimum-temperature difference that exists between models and data. The effect of the lake upon regional climate is significant and proves to be critical to reproducing the early Eocene climate of North America.