We have performed an overnight single dose dexamethasone suppression test in 15 children at different dosages to determine the minimum dose required to suppress the 0800 h serum cortisol level. With a dexamethasone dose of 0.1 mg/m2 two of seven children showed suppression. When the dose was increased to 0.3 mg/m2 all of six patients showed suppression as did three given dexamethasone 0.5 mg/m2. We recommend that doses of dexamethasone should be calculated in terms of body surface area when assessing the hypothalamo-pituitary axis in children and that a dose of 0.3 mg/m2 should be used for suppression tests.