Patients with primary adrenal insufficiency (PAI) require increased doses of glucocorticoids and mineralocorticoids during stressors, such as surgery, trauma, and sepsis. Although current guidelines exist for dose adjustments in these situations, there is no accepted dosing regimen for patients with PAI participating in intensive endurance exercise. Given the extensive physiologic stress of events, such as marathons, triathlons, and similar events, it is likely that a “stress-dose” of adrenal replacement therapy will not only prevent adrenal crisis, but also improve performance. A 50-year-old male endurance athlete with known PAI reported severe fatigue, nausea, and malaise after competing in prior marathons and intensive endurance exercise. After supplementing with glucocorticoids and mineralocorticoids before competition, he experienced decreased symptoms and improved performance. To better care for these patients, further studies should be conducted to provide safe and effective glucocorticoid and mineralocorticoid dose adjustments before intensive endurance exercise.