P450 oxidoreductase (POR) facilitates electron flux to type 2 microsomal P450 cytochrome enzymes (CYPs), including the adrenal steroidogenic enzymes CYP17A1 and CYP21A2. Due to the combined impairment of these enzymes, POR deficiency (PORD), an autosomal recessive condition, results in congenital adrenal hyperplasia characterised by combined glucocorticoid and postnatal sex steroid deficiency. This study focuses on urinary steroid excretion in infants affected by PORD in the first week of life. We report on three neonatal PORD cases from two families. One family had two affected babies born three years apart who were stillborn and first-day deceased, respectively. DNA sequencing revealed a homozygous 3 bp deletion in exon six leading to an glutamic acid deletion (p.[Glu217del]). Bladder contents were obtained from the stillborn baby, and excreted urine was obtained from the second baby. In a second family, their second affected newborn, antenatally diagnosed carrying the common homozygous p.(Ala287Pro) mutation, had urine collected daily during the first week of life. Steroid excretions were quantified by gas chromatography-mass spectrometry (GC-MS). The birth-day excretions were very similar in all babies. Most notable and unusual was a large excretion of unmetabolised corticosterone, suggesting inhibited catabolism to allow maximum active gluco- and mineralocorticoid availability at birth. Because CYP3A7 (16α-hydroxylase) requires POR, there was an almost complete absence of usually dominant 3β-hydroxy-Δ5 steroids (16α-OH-DHEA and 16α-OH-pregnenolone) and the usually characteristic precursor pregnenolone metabolite 5-pregnene-3β,20α-diol (pregnenediol, 5PD). In the baby sequentially studied over a week, we observed gradual maturation to the typical and familiar PORD neonatal metabolome. At the end of the period, the minimally catabolised corticosterone had diminished, and steroid excretion was completely dominated by 5PD, excreted as both mono- and disulphate conjugates. Whether this metabolome is distinctive of all PORD infants, not just those with severe manifestation, is not known. On the first day of life, standard diagnostic markers are compromised due to fetal-placental-maternal contribution and unique neonatal steroid metabolism. However, the Day 1 PORD steroid metabolome remains distinctive, and we propose using additional biochemical markers reflective of the near complete reduction of POR-dependent CYP3A7 (16α-hydroxylase) activity to improve diagnostic yield.