Differentiating physiological adaptation from cardiac pathology can prove challenging in athletes, as the nature and magnitude of cardiac remodelling in response to exercise are governed by several important factors. These factors include age, sex, anthropometry, ethnicity and sporting discipline.1One element not routinely considered in the athlete’s heart continuum, is the impact of repeated exposure to hot environmental conditions. Several major sporting events take place in the summer months, often in hot ambient conditions (e.g.2004 Athens, 2008 Beijing and 2016 Rio Olympics, 2015Beijing and 2019 Doha IAAF World Championships).The 2020 Tokyo Olympics are predicted to be the hot-test on record. Exercising in the heat induces thermo-regulatory and physiological strain,2which in turn lead to reductions in endurance exercise capacity.3Athleteswill accordingly train repeatedly in a hot environment prior to a competition as a method to offset these potential performance impairments.4Most natural acclimatisation (or artificial acclimation) protocols require athletes to undertake daily exposure to the heat for up to 14 days, in an attempt to increase whole-body (core and skin) temperature, induce sweating and increase skin blood flow.5Heat acclimation, also induced by passive heating, elicits cardiovascular adaptations allowing for a lowered heart rate, an increased stroke volume, a better sustained_Q, a better defended blood pressure and an increased myo-cardial efficiency and compliance while exercising in theheat.6However, the repercussions of repeated heat exposure on cardiac structure remain unknown. Accordingly, we aimed to isolate the influence of repeated heat exposure on the myocardium, without the confounding factor of exercise, by examining the impact of 12 days of passive heat acclimation (PHA)on the resting cardiac structure and function of well-trained male athletes.