Recent research suggests that women tend to exhibit less of a precipitous decline in run velocity during the latter stages of a marathon than men when the covariates of age and run time are controlled for. The purpose of this study was to examine this sex effect with the added covariate of heat stress on pacing, defined as the mean velocity of the last 12.2 km divided by the mean velocity of the first 30 km. A secondary purpose of this investigation was to compare the pacing profiles of the elite men and women runners and the pacing profiles of the elite and nonelite runners. Subjects included 22,990 men and 13,233 women runners from the 2007 and 2009 Chicago marathons for which the mean ambient temperatures were 26.67° C and 2.77° C, respectively. Each 5-km split time was measured via an electronic chip worn on the participants' shoe. Multiple regression analysis indicated that age, sex, heat stress, and overall finish time (p < 0.01 for each) were simultaneous independent elements of pacing. Nonelite women were consistently better pacers than nonelite men in both marathons, and this sex difference was magnified from cold to warm race temperatures. No difference (p < 0.05) in pacing was found between elite men and women runners. Elite men and women had enhanced pacing over their nonelite counterparts. In hotter temperatures, coaches of novice runners should advise their athletes to implement a slower initial velocity to maintain or increase running velocity later in the race.