Isolated adrenocorticotropic hormone (ACTH) deficiency (IAD) is a rare cause of adrenocortical insufficiency, especially in children, and may be an underestimated cause of neonatal death. Early postnatal diagnosis may prevent hypoglycemic seizures, Addisonian crises, and death. There are also occasional reports of prenatal diagnosis of IAD by findings on the maternal triple-marker screen (TMST), a combined serum analyte test that measures levels of alpha-fetoprotein, human chorionic gonadotropin, and unconjugated estriol for the detection of Down syndrome and open neural-tube defects. An isolated low estriol level is usually correlated with compromised uteroplacental perfusion and frequently associated with fetal death. A low estriol level in the context of normal fetal sonography and growth, after exclusion of placental sulfatase deficiency and Smith-Lemli-Opitz syndrome, should raise the suspicion of deficient fetal steroidogenesis, which leads to decreased production of adrenal dehydroepiandrosterone sulfate. We describe 2 brothers with adrenal insufficiency resulting from IAD. The parents are first cousins whose first son is healthy. During the pregnancy of the second son, who died at the age of 7 weeks as a result of presumed cardiomyopathy, a low estriol level on the TMST was ignored because of a normal fetal ultrasound. In the third pregnancy, a low level was found again, and the mother was referred to our tertiary center. Ultrasonography revealed no abnormalities, and karyotype was normal. Normal levels of steroid sulfatase activity and 7-dehydrocholesterol ruled out X-linked ichthyosis and Smith-Lemli-Opitz syndrome, respectively. Postnatally, basal and stimulated cortisol and ACTH levels were low. Other pituitary functions were normal, suggesting the diagnosis of IAD. The patient was treated with a stress dose of hydrocortisone on day 2 of life, which was tapered to a maintenance dose. At the time of this writing, he was 7 months old, with normal growth and development. Recently, loss-of-function mutations in the human TPIT gene were detected in autosomal recessive IAD. TPIT is a cell-restricted T-box transcription factor that is important for the terminal differentiation of pituitary corticotrophs. Therefore, we performed molecular analysis of the TPIT gene, which revealed a new mutation (IVS4+1G>A) that affects the first nucleotide of the splice site at the 5' end of the fourth intron. This stop codon probably leads to loss of TPIT function by nonsense-mediated mRNA decay, as it does for other TPIT nonsense mutations. We recommend that pregnant women with an isolated low estriol level of unexplained etiology be referred for additional evaluation by a multidisciplinary team that includes a geneticist and pediatric endocrinologist. Prompt ACTH testing in the first postnatal days will allow for early diagnosis. The immediate institution of glucocorticoid therapy, with proper instructions for stress management, can prevent unnecessary neonatal death secondary to an easily treatable disease.