Abstract Context and Objective We describe the clinical features and biochemical characteristics of a unique population of severely affected untreated patients with congenital adrenal hyperplasia (CAH) from an Indonesian population with proven cortisol deficiency but without clinical signs of cortisol deficiency. We evaluated the in vitro glucocorticoid activity of all relevant adrenal steroid precursors occurring in patients with CAH. Design Cross-sectional cohort study and translational research. Intervention/Main Outcome Measures Adrenal steroid precursor concentrations before and 60 minutes after ACTH administration to 24 untreated patients with CAH (3 to 46 years) with proven cortisol deficiency (<500 nmol/L post-ACTH) measured by liquid chromatography–tandem mass spectrometry were compared with six control patients (Mann-Whitney U test). Glucocorticoid receptor (GR) activation was determined by dual-luciferase assays in human embryonic kidney cells transfected with the GR and exposed to increasing amounts of adrenal steroid precursors for 24 hours. Results Blood concentrations of the steroid precursors 11-deoxycortisol (457 nmol/L, P = 0.003), 11-deoxycorticosterone (55 nmol/L, P = 0.003), 17-hydroxyprogesterone (610 nmol/L, P < 0.001), progesterone (29 nmol/L, P < 0.001), and 21-deoxycortisol (73 nmol/L) were strongly elevated compared with control subjects. The GR was activated with comparable potency to cortisol by corticosterone and 21-deoxycortisol or with 4 to 100 times lower potency by 11-hydroxyprogesterone, 11-deoxycortisol, aldosterone, 11-deoxycorticosterone, progesterone, and 17-hydroxyprogesterone. Conclusions We identified strongly elevated adrenal steroid precursor concentrations in blood from untreated patients with CAH and demonstrated glucocorticoid activity of these adrenal precursors in vitro, suggesting a possible role of these precursors in the clinical phenotype of these patients. Further studies are necessary to evaluate the role of these precursors in more detail.