PURPOSE: In vitro studies on platelet function have shown that lower temperatures encourage aggregation and overexpression of the adhesion molecules. Anesthesia guidelines recommend the prevention of hypothermia (T < 36 º). This study aims to explore the impact of intraoperative body temperature on venous thromboembolism (VTE) and flap pedicle thrombosis within 30 days of microsurgical free tissue transfer surgery to the head and neck. METHODS: All patients who underwent head and neck reconstruction with free tissue transfer from January 2012 through March 2022 in a tertiary center were included in this study. Data were collected from a retrospective and prospective held database. Intraoperative temperature readings were extracted electronically in one-minute intervals for each surgery. Non-physiologic readings less than 30 or more than 42 were discarded. A median was calculated for every five consecutive readings to create a five-minute temperature reading. A median value under 36 º was considered an onset of hypothermia. VTE events and thrombosis of the free flap pedicle (arterial or venous) that require revision surgery were the study outcomes, and they were recorded within 30 days of the index surgery. RESULTS: 895 patients were included, and out of those n=300, 34% were females. The mean intraoperative body temperature and Caprini score for the whole cohort were 36.13 ± 1.08 º and 6.54 ± 1.74. The VTE and pedicle thrombosis rates were (n=37, 4.0%) and (n=58, 6%), respectively. The incidence of patients developing at least one onset of hypothermia was (n=720, 80%). The mean number of hypothermia onsets and the mean length of surgery were 52.40 ± 84.80 and 685.12 ± 144.31 minutes, respectively. The mean for Caprini score in those who developed at least one onset of hypothermia did not differ significantly from those who were normothermic (≥ 36 º) for the whole surgery (6.55±1.76 vs. 6.46±1.60, p=0.533). The rate of VTE in those who experienced at least one onset of hypothermia was significantly higher than in those who were normothermic for the whole surgery (n=35/720, 3.9% vs. n=2/175, 0.22%, p=0.026). This association persisted even after controlling for the Caprini score, the number of hypothermic onsets, and the length of surgery in a multivariable regression model (OR, 5.478, 95%CI: 1.222-24.557). In contrast, the rate of free flap pedicle thrombosis was lower in patients who experienced hypothermia than others in the cohort (n=9/175, 1.01% vs. n=49/720, 5.47%). However, the difference failed to reach a statistical significance (p=0.422). CONCLUSIONS: Hypothermia onset is associated with an increased rate of VTE postoperative, even after controlling for the number of hypothermia onsets, Caprini score, and length of surgery. Patients who experienced hypothermia during surgery had a lower incidence of free flap pedicle thrombosis than those who were normothermic throughout the surgery. However, no conclusion can be made about the impact of hypothermia on pedicle thrombosis since the difference failed to reach statistical significance.