Unplanned perioperative hypothermia is a well-known complication to anesthesia. This study compares esophageal and nasopharyngeal temperature measured in the same patient for a period of 210 minutes of anesthesia. Forty-three patients undergoing colorectal surgery were randomly assigned in 2 groups, with or without a prewarming period (group A = prewarming [n = 21] or group B = no prewarming [n = 22]). Demographics were similar in both groups. Mean temperatures at 210 minutes were statistically different between the groups at both sites of measurement. Esophageal temperature in group A was 36.5 ± 0.6 vs 35.8 ± 0.7 in group B (P = .001), and nasopharyngeal temperature was 36.7 ± 0.6 and 36.0 ± 0.6 in group A and group B, respectively (P = .002). A negative correlation was found between esophageal temperature and age (r2 = -.381, P < .012). Esophageal temperature was different with respect to BMI below or above 25. The temperatures were 35.81 ± 0.66 in the lower BMI group vs 36.46 ± 0.59 (P < .001). These results demonstrate a difference between the 2 measurement techniques and that prewarming, age and BMI have an impact on measured temperatures.