Aim: We aimed to quantify the impact of a raised preoperative ambient temperature (Tambient) on core temperature (Tcore) after induction of anesthesia in children. Background: It has been suggested that prewarming of patients before anesthesia induction reduces postinduction drop in Tcore. Neither the prewarming temperature nor its duration is established for adults or children. Nevertheless, it remains common practice to either warm the operating theatre and induction room or employ radiant heaters prior to induction of anesthesia, particularly for infants and neonates. We aimed to quantify the benefit, if any, of this warming practice. Methods: We conducted a prospective clinical study to assess Tcore behavior in children randomized to either raised or standard ambient temperature as a prewarming technique prior to induction and until the operation commenced. We have called this ‘preoperative’ warming. Well, children scheduled for elective surgery where presurgical anesthetic duration exceeded 20 min were randomized to a Tambient of either 26 or 21°C. Esophageal temperature was monitored continuously until the operative procedure commenced. Results: There were 30 children in each group. Those in the warmed group (26°C) had a statistically significant higher initial Tcore (0.4°C warmer) and less drop in their Tcore (0.18°C benefit at 20 min). Although younger/lighter/shorter individuals were more likely to drop their Tcore, a warmer Tambient had only 0.1°C thermal benefit irrespective of age. Conclusions: There are statistically significant thermal advantages to preoperative environmental warming. This study provides data to assist the anesthetist in deciding when these are likely to be clinically relevant.