At the request of the City of Fort Collins Utilities and local building officials, the Colorado Climate Center recently updated calculations of heating and cooling design temperatures for Fort Collins, Colorado. Using a combination of hourly observations through 1994, hygrothermograph records in the mid 1990s, and data from automated temperature instruments since 1996, a complete time series of hourly temperatures was assembled for the past 38 years. Design temperatures for heating and cooling (hot and cold temperature thresholds that are exceeded only a small percentage of the hours over a several year time period) were then calculated and compared to results from previous ASHRAE reports. When period-of-record data were used going back to the 1960's, the new calculations of heating and cooling design temperatures had not changed significantly. However, when only the data for the past 10-20 years were included, then both winter heating and summer cooling design temperatures showed substantially higher values than those recommended from traditional ASHRAE reports. This paper shows the recent warming trends observed at Fort Collins, CO in both summer and winter and the upward trend in computed design temperatures. A discussion of the implications of changing design temperatures will follow as well as what long-term climate data may suggest.