Congenital adrenal hyperplasia (CAH) describes a group of autosomal recessive disorders of steroid biosynthesis, in 95% of cases due to 21-hydroxylase deﬁciency. The resulting hormonal imbalances lead to increased 17-hydroxyprogesterone and androgens levels, at the expense of decreased concentrations of glucocorticoids and, in some cases, of mineralocorticoids. A variety of clinical presentations accompany a range of severities, which are described as different forms of CAH, and are the result of these hormonal imbalances. The incidence of CAH worldwide is approximately 1 in 15,000 live births, and is population-dependent / thus, its inclusion in neonatal screening tests is widely discussed. Diagnosis of CAH is based on the quantiﬁcation of 17-hydroxyprogesterone, usually by immunoassay, which has low speciﬁcity and high false-positive rates, resulting in a relatively high demand for a second-tier conﬁrmation test. We report a case of a newborn recognized as female at birth, but showing ambiguous genitalia and other CAH clinical features, including hypernatremia, in the first days of life. In addition to the classical assays, liquid chromatography&ndash / tandem mass spectrometry was used to determine the serum steroid profile, allowing for the accurate and simultaneous quantification of seven steroids in the same analysis. Such an application immediately revealed an alteration in the levels of specific steroids related to CAH, leading to an early intervention by hormone replacement therapy. Subsequently, the diagnosis of classic CAH due to 21-hydroxylase deﬁciency was further confirmed by molecular testing.