Warm-season (1 May-30 September) hourly dewpoint data were examined for temporal changes at two weather stations in northeastern Illinois during a 42-yr period (1959-2000). This area has dense population (greater than 8 million), and shifts to more or less atmospheric moisture have major implications on cooling demands. The 42-yr period was analyzed as two separate arbitrarily chosen equally sized periods, the early (1959-79) and the later (1980-2000) periods. Analyses of data from Chicago's O'Hare International Airport and the Greater Rockford Airport showed a statistically significant increase in the number of hours with dewpoints greater than or equal to 24ЊC (an important cooling-plant threshold) in the latter period. Examination of heatwave periods indicated that later (especially 1995 and after) heat waves contained many more extreme dewpoint values. These increases in extreme dewpoint characteristics in northeastern Illinois affect the operation of, and suggest shifts in design criteria for, airconditioning systems and affect summer peak electrical loads.