The impact of climate warming on Swiss building energy demand was investigated by means of the degree-days method. A procedure to estimate heating degree-days (HDD) and cooling degree-days (CDD) from monthly temperature data was developed, tested and applied to four representative Swiss locations. Past trends were determined from homogenized temperature data for the period 1901-2003. The range of possible future trends for the 21st century was assessed based on 41 regional climate change scenarios derived from 35 simulations with 8 global climate models. During 1901-2003, the HDD were found to have decreased by 11-18%, depending on the threshold temperature (8, 10 or 12 degrees C) and location. For the period 1975-2085, the scenario calculations suggested a further decrease between 13% and 87%. For CDD, accelerating positive trends were found during the 20th and 21st centuries. The HDD showed the largest absolute and the CDD the largest relative sensitivity to warming (albeit starting from relatively low levels). Weather data currently used for building design increasingly lead to an overestimation of heating and underestimation of cooling demand in buildings and, thus, require periodic adaptation. Projections were particularly sensitive to the choice of temperature scenario. Nevertheless, they suggest for the next decades significant, seasonally and regionally variable shifts in the energy consumption of Swiss buildings that deserve further study. In particular, greater attention needs to be paid in future to the summer thermal behaviour of buildings. (c) 2005 Elsevier Ltd. All rights reserved.