BACKGROUND A 4-week course of high-dose glucocorticoids may cause prolonged adrenal suppression even after a 9-day tapering phase. In this study, adrenal function and signs and symptoms of adrenal insufficiency were prospectively assessed in children with acute lymphoblastic leukemia (ALL) after induction treatment including high-dose prednisone (PDN) or dexamethasone (DXM). METHODS Sixty-four children with ALL, treated according to the AIEOP ALL 2000 Study protocol, underwent low dose ACTH (LD-ACTH) stimulation 24 hr after the last tapered steroid dose. In those with impaired cortisol response, additional LD ACTH tests were performed every 1-2 weeks until cortisol levels normalized. Signs and symptoms of adrenal insufficiency were recorded during the observation period. RESULTS All patients had normal basal cortisol values at diagnosis. Twenty-four hours after last glucocorticoid dose, morning cortisol was reduced in 40/64 (62.5%) patients. LD-ACTH testing showed adrenal suppression in 52/64 (81.5%) patients. At the following ACTH test 7-14 days later, morning cortisol values were reduced in 8/52 (15.4%) patients and response to the test was impaired in 12/52 (23%). Adrenal function completely recovered in all patients within 10 weeks. No difference was found between patients treated with PDN or DXM. Almost 35% of children with impaired cortisol values at the first test developed signs or symptoms of adrenal insufficiency. One child developed a severe adrenal crisis during adrenal suppression. CONCLUSIONS High-dose glucocorticoid therapy in ALL children may cause prolonged adrenal suppression and related clinical symptoms. Laboratory monitoring of cortisol levels and steroid coverage during stress episodes may be indicated.