ACTH stimulation is the standard test for assessment of adrenal function. It was suggested that the low dose (1 microg) would be more sensitive for detecting mild secondary adrenal insufficiency than the usual dose of 250 microg. The aim of this study was to find the optimal diagnostic criteria and to compare standard dose test (SDT) with the low dose test (LDT). A group of patients treated with corticosteroids for the 6 months was considered to have hypothalamo-pituitary-adrenal impairment. Studies were performed in 14 corticosteroid-treated and 28 control subjects in random order on 2 consecutive days. Tests were analyzed using the receiver operating characteristic curve method. The best test was cortisol increment at 15 min of the LDT. It was significantly better than the cortisol concentration at 15 min of the SDT, the best test during the SDT (receiver operating characteristic curve area and 95% confidence interval: LDT, 0.997 and 0.956-0.999; SDT, 0.827 and 0.662-0.929; P = 0.0113). For the cortisol increment at 15 min of the LDT at 100% sensitivity, the diagnostic value was 100 mmol/L, and the specificity was 96%. Therefore, the LDT is superior to the standard dose test in the assessment of secondary adrenal insufficiency.