Background: The energy cost of running per unit mass and distance (Cr) is reported to increase or to remain constant after a fatigue test in young healthy adults. No studies were performed on master athletes about this topic. Research question: Does the energy cost of running increase after a half marathon in a group of master athletes? Type of study: Pre-Post single group design. Methods: Ten healthy master athletes took part in the experiment. The experimental protocol involved an incremental test to exhaustion, a half marathon simulation and two sessions of energy cost assessments before and after the race simulation. Results: Neither comparing fatigue to no-fatigue conditions (p>0.05) nor as a function of the running speed (p>0.05) could a change in the group mean energy cost be revealed. Conclusions: The energy cost of running is not affected by a half marathon race within a group of master athletes.