The temporal distribution of surface atmospheric water vapor levels during short-duration extreme heat events at Chicago was assessed. Heat events were characterized by at least three consecutive days with a minimum temperature greater than 24°C (75°F) and a maximum temperature greater than 35°C (95°F). Thirteen such heat events were identified during the 75-year (1928-2002) record at Chicago Midway Airport located inside the city. Average dew points in events fluctuated considerably over time, but generally increased. Event average hourly dew point values before the mid 1980s were generally below 21°C, rising to an average of 24°C during the mid 1990s. The temporal changes in dew point values were related to two factors: 1) the existence of average to above-average regional precipitation anomalies occurring in May and during the 20-day period prior to the heat event, and 2) changes in agricultural practices that have led to enhanced evapotranspiration rates in the Midwest. Changes in dew point levels associated with short-duration heat events impact air conditioning system design criteria and influence the maximum energy demand placed upon utilities.