© 2021 John Wiley & Sons LtdBackground: Unintended perioperative hypothermia, defined as core body temperature less than 36°C, is closely related to many complications such as cardiovascular diseases, peroperative hemorrhagic diathesis and impairment of drug metabolism. Determination of the incidence of perioperative hypothermia and examination of risk factors may help to prevent hypothermia and its complications. Methods: We conducted a prospective, observational study of 2015 patients who underwent various operations under general anesthesia, and the risk factors of developing hypothermia were analysed. Results: The incidence of perioperative hypothermia was 78.6%. The incidence of hypothermia within 2 hours was 56.6%, and after 2 hours, it was 100%. Mean age was 49.36 ± 16.10, and 17.8% were over 65 years old. The mean body mass index (BMI) was 27.96 ± 3.94 kg/m2; 60.8% of the patients had American Society of Anesthesiologists (ASA) I score, 33.4% had ASA II and 5.8% had ASA III; 35.8% of the patients had co-morbidities. Intravenous and irrigation fluids were unwarmed; 99.9% of the patients were warmed passively, and only 0.1% of patients received active heating intraoperatively. The incidence of hypothermia was higher in overweight (BMI ≥ 25 kg/m2), elderly (>65 years) patients and those with co-morbidities. High American Society of Anesthesiologists (ASA) scores, grade 3-4 surgery, endoscopic surgery, duration of anesthesia >2 hours, infusion or irrigation >1000 mL significantly increased the incidence of hypothermia. Conclusion: The incidence of perioperative hypothermia found was high. Important risk factors were found as prolonged duration of anesthesia and surgery, advanced age, overweight, high ASA scores, major surgeries, endoscopic operations and unwarmed fluid administration. High incidence may be reduced by raising awareness, considering fossible risk factors and following the recommendations of the guidelines on prevention of perioperative hypothermia.