Objectives We aimed to explore the impact of an ultra-endurance race on continuously measured glycaemia and to understand potential physiological mechanisms, as well as the consequences for performance and behavioural alertness. Methods Fifty-five athletes (78% men, 43.7 +/- 9.6 years) ran a 156-km ultra-trail race (‘Trail Scientifique de Clécy’: six 26-km laps, total elevation +6000 m). Participants wore a masked continuous glucose monitoring sensor from the day before the race until 10 days post-race. Blood was taken at rest, during refuelling stops after each lap, and after 24-h recovery. Running intensity (% heart rate reserve), performance (lap times), psychological stress and behavioural alertness were explored. Linear mixed models and logistic regressions were carried out. Results No higher risk of hypo or hyperglycaemia was observed during the exercise phases of the race (i.e., excluding stops for scientific measurements and refuelling) compared with resting values. Laps comprising a greater proportion of time spent at maximal aerobic intensity were nevertheless associated with more time >180 mg/dL (P=0.021). A major risk of hyperglycaemia appeared during the 48-h post-race period compared with pre-race (P180 mg/dL during recovery vs. 5.5% during resting. Changes in circulating insulin, cortisol and free fatty acids followed profiles comparable with those usually observed during traditional aerobic exercise. However, creatine phosphokinase, and to a lesser extent lactate dehydrogenase, increased exponentially during the race (P<0.001) and remained high at 24-h post-race (P<0.001; respectively 43.6 and 1.8 times higher vs. resting). Glycaemic metrics did not influence physical performance nor behavioural alertness. Conclusion Ultra-endurance athletes were exposed to hyperglycaemia during the 48-h post-race period, possibly linked to muscle damage and inflammation. Strategies to mitigate muscle damage or subsequent inflammation before or after ultra-trail races could limit recovery hyperglycaemia and hence its related adverse health consequences.