Classic congenital adrenal hyperplasia (CAH) is associated with impaired function of the adrenal cortex and medulla leading to decreased production of cortisol and epinephrine. As a result, the normal exercise-induced rise in blood glucose is markedly blunted in such individuals. We examined whether an extra dose of hydrocortisone, similar to that given during other forms of physical stress such as intercurrent illness, would normalize blood glucose levels during exercise in patients with CAH. We studied hormonal, metabolic, and cardiorespiratory parameters in response to a standardized high-intensity exercise protocol in nine adolescent patients with classic CAH. Patients were assigned to receive either an additional morning dose of hydrocortisone or placebo, in addition to their usual glucocorticoid and mineralocorticoid replacement in a randomized, double-blind, crossover design 1 h before exercising. Although plasma cortisol levels approximately doubled after administration of the additional hydrocortisone dose compared with the usual single dose, fasting and exercise-induced blood glucose levels did not differ. In addition, no differences were observed in the serum concentrations of the glucose-modulating hormones epinephrine, insulin, glucagon, and GH and of the metabolic parameters lactate and free fatty acids. Although maximal heart rate was slightly higher after stress dosing (193 +/- 3 vs. 191 +/- 3 beats/min, mean +/- sem, P < 0.05), this did not affect exercise performance or perceived exertion. We conclude that patients with classic CAH do not benefit from additional hydrocortisone during short-term, high-intensity exercise. Although this has not been tested with long-term exercise, a high degree of caution should be used when considering the frequent use of additional hydrocortisone administration with exercise, given the adverse side effects of glucocorticoid excess.