Background: Adrenal insufficiency has been reported in 46 % to 81.5 % of children receiving corticosteroids for acute lymphoblastic leukemia (ALL). Methodology: To assess the frequency of such insufficiency, 40 patients under 18 years (mean: 8.5 years) with new diagnosis of ALL were studied. Base-line cortisol and adrenocorticotropin (ACTH) levels were measured, and they received 5-week therapy with prednisolone. After 3 days off-steroid therapy, a stimulation test with ACTH 1 μg was done. In patients with abnormal cortisol (<18 μg/dL) new ACTH tests were done and cortisol levels were determined at days 7, 14 and 30 until cortisol post-stimulation levels were normal. Results: Three days after the last steroid dose 29/40 (72.5 %) had adrenal insufficiency after ACTH stimulus. At day 30 no one had abnormal cortisol levels after ACTH stimulus. All patients with adrenal suppression were over 5 years (HR 4.69; CI95 %: 1.44-15.32; p = 0.011). Conclusion: Steroids used during ALL treatment may cause adrenal insufficiency. Patients over 5 years are at high risk of developing adrenal suppression. We suggest to follow-up those patients with stress episodes after induction chemotherapy as steroid supplementation may be indicated.