The graded benefit of exercise on cardiovascular risk is well established. Observational studies have, however, focused on mild to moderate levels of activity. In Australia, in excess of 30% of the adult population exercise in excess of the upper range as studied in the literature. There is limited evidence as to whether the risk/benefit ratio is maintained in those participating in regular intense exercise. Case reports and circumstantial evidence raise the possibility that more extreme exercise may have some detrimental effects. In effect, exercise may have a typical dose-response curve with a plateau or even toxicity at more extreme levels. Given the increasing popularity of endurance sporting pursuits, there is a need for further research into the cardiac adaptations and consequences of extreme habitual exercise.