We report on a brother and sister with adrenal insufficiency due to isolated adrenocorticotropin hormone deficiency discovered in the neonatal period. The first-born, a male infant, died; pathological findings suggested bilateral adrenal hypoplasia transmitted as an autosomal recessive trait. Plasma estriol levels were assayed during the mother's next pregnancy. The prenatal diagnosis allowed immediate and effective management of the second affected child. The supplementary evidence from the endocrine findings, unavailable on her brother, enabled us to make a diagnosis of isolated central ACTH deficiency. As the defect was found in infants of both sexes in the same family, it is in all likelihood transmitted as an autosomal recessive trait. We consider it important for genetic counselling to perform autopsies on all newborn infants whose death has no apparent cause. Maternal plasma estriol assays during pregnancy can help diagnose fetal adrenal insufficiency, whether the defect is central or adrenal.