▼ Research has suggested that faster, women, and older runners are more likely to run at a consistent pace during marathon races. Therefore, the purpose of this study was to determine the simultaneous influences of age, sex, heat stress and finish time on marathon pacing. Pacing was defined as the mean velocity of the last 12.2 kilometers divided by the mean velocity of the first 30 kilometers. Subjects included 22,990 men and 13,233 women runners from the 2007 and 2009 Chicago marathons. The average ambient temperatures during the 2007 and 2009 marathons were 26.67 °C and 2.77 °C, respectively. Multiple regression analysis indicated that age, sex, heat stress, and overall finish time (p<0.01 for each) were simultaneous independent elements of pacing. Women were consistently better pacers than men in both marathons though the gender difference increased from cold to warm race temperatures. Coaches and runners can use these findings to improve the likelihood for more optimal pacing. Advisors/Committee Members: Vanderburgh, Paul.