We investigated whether a cooling vest worn during an active warm-up enhances 5-km run time in the heat. Seventeen competitive runners (9 males, VO_2max = 66.7 ± 5.9 ml/kg/min, 8 females VO_2max = 58.0 ± 3.2 ml/kg/min) completed two simulated 5-km runs on a treadmill following a 38-min active warm-up during which they wore either a t-shirt (C) or a vest filled with ice (V) in a hot, humid environment (32° C, 50% RH). Wearing the cooling vest during warm-up significantly (p < 0.05) blunted increases in body temperature, heart rate (HR) and perception of thermal discomfort during warm-up compared to control. At the start of the 5-km run, esophageal, rectal, mean skin, and mean body temperatures averaged 0.3, 0.2, 1.8 and 0.4° C lower; HR averaged 11 bpm lower; and, perception of thermal discomfort (5-point scale) averaged 0.6 point lower in V than C. Most of these differences were eliminated during the first 3.2 km of the run and these variables were not different at the end. 5-km run time was significantly lower (p < 0.05) by 13 s in V than C, with a faster pace most evident during the last two-thirds of the run. We conclude that a cooling vest worn during active warm-up by track athletes enhances 5-km run performance in the heat. Reduced thermal and cardiovascular strain, and perception of thermal discomfort in the early portion of the run appear to permit a faster pace later in the run.