During ultra-endurance events, thermoregulation plays an important role in guaranteeing physical performance and avoiding health problems (i.e: heat stroke). Hyperthermia occurs when the core temperature rises above 38.5°C. Weather conditions (heat & humidity) or training level of athletes may affect thermoregulation. PURPOSE: To measure core temperature (Tcore) in high-level triathletes during the Ironman World Championship (Hawaii) under thermal stress conditions. METHOD: Tcore of fifteen triathletes (age: 36.11±7.36 years, weight: 71.14±7.12 kg, height: 179± 0.04 cm, fat %: 8.48±0.85) who classified for the Ironman World Championship in Kailua - Kona were measured using an ingestible pill telemetry system before competition, upon completion of the event (marathon) and after 60 minutes of recovery. Event mean ambient temperature was 29.1 °C (range 20–36 °C) and relative humidity was 76% (range 71-88%). Body Mass Index (BMI) and exercise perceived exertion (Borg Scale and Visual Analog Scale-Pain) were measured before the event and 60 minutes after the event. Performance variables were extracted from their own GPS and official race time and splits. Statistical analysis was performed using Repeated Measured Test, ANOVA. RESULTS: Finish time (hh:mm:ss) was 10:06:56 ± 0:48:30. Initial Tcore was 36.62 ± 0.17 °C, increasing at the end of the event (38.55 ± 0.64 p<0.01) and remaining high 60 minutes after the event (38.65 ± 0.41°C p<0.002). BMI significantly decreased after the event (22.85 ± 1.11 vs. 21.73 ±1.36 p<0.05) while exercise perceived exertion - Borg Scale (10.2 ± 1.64 vs. 18.60 ± 1.67 P<0.003) and perceived muscle pain - VAS Pain (2.75± 1.59 vs. 9.08 ± 1.13 p<0.001) increased significantly after the event. CONCLUSION: High-level triathletes competing under thermal stress conditions in the Kona Ironman reached a state of hyperthermia during the event. After 60 minutes of recovery, when exercise perceived exertion was at a maximum, the hyperthermic state persisted. Strategies to aid cooling and recovery, especially after the race, should be considered by both triathletes and event organizers. Supported by Government of Extremadura-Spain (GR15020 - CTS036) and 226ERS- Endurance Research Lab