OBJECTIVES: Our purpose was to determine the incidence and etiology of hypothermia during laparoscopic surgery and to evaluate the role of irrigation fluid temperature. STUDY DESIGN: A prospective randomized study was performed of 35 women undergoing operative laparoscopy under general anesthesia who received surgical irrigation fluid either at ambient temperature or warmed to 39°C. The core body temperature was determined with use of both an esophageal sensor and a tympanic membrane sensor and was expressed as the change from baseline. Additional data collected included age, height, weight, amount of irrigation fluid and intravenous fluid used, room temperature, length of anesthesia, and amount of carbon dioxide used for pneumoperitoneum. RESULTS: Hypothermia occurred in 94% of all patients, with no difference in incidence between the groups. The minimal core temperature was lower in the ambient temperature group (-1.7°± 0.2°C) than in the warmed fluid group (-1.0°± 0.2°C). Of the variables measured, length of anesthesia and the amount of ambient temperature fluid alone explained the drop in core temperature. CONCLUSION: Hypothermia is extremely common in laparoscopic surgery and is related to the length of anesthesia and the use of ambient temperature irrigation fluid. The use of warmed irrigation fluid can decrease, but not eliminate, this drop in core temperature.