Background: Congenital Adrenal Hyperplasia (CAH) comprises a spectrum of autosomal recessive diseases, resulting in enzymatic defects in the cortisol secretion. CAH newborn screening can avoid neonatal mortality in children with the salt-wasting form and prevent incorrect gender assignments in females. The occurrence of false-positive results creates diagnostic difficulties presenting therapeutic implications. Beckwith Wiedemann Syndrome (BWS) is a congenital disease characterized by somatic overgrowth, increased risk of neonatal hypoglycemia, and development of embryonic tumors. BWS is due to (epi)genetic changes involving growth-regulating genes with good genotype-phenotype correlation. The adrenal gland is frequently involved and may present diffuse cytomegaly of the adrenal cortex1. We reported a BWS newborn girl with a false-positive diagnosis of CAH in the screening. Case report: The patient was born at 39 weeks from an uneventful cesarean section, 5.6kg (>p97) and 52cm (>p97), referred to the Endocrinology service due to abnormal neonatal tests (neonatal 17-OHP: 96ng/mL) collected at 6 days old. At 14 days old, she was 6.3 kg (Z:+5.59), and 58cm (Z:+2.47), BMI: 18.7 kg/m2 (Z:+4.45), and with typical female external genitalia, ruling out the diagnosis of classic CAH. She presented some syndromic characteristics as macroglossia, ogival palate, orbital hypertelorism, hepatomegaly, and umbilical hernia. At 1 month and 14 days old, serum 17OHP was 7.4ng/mL, androstenedione: 6.1 ng/mL, total testosterone: 279ng/dL, 11-deoxycortisol: 2.11ng/mL, cortisol: 5.0ug/dL, and ACTH: 54pg/mL. At five months old she evolved with normalization of serum 17OHP, androstenedione and testosterone levels (1.36ng/mL, <0.50ng/mL, and 37ng/dL, respectively), but still with high DHEAS levels: 2913ng/mL. At 11 months old, DHEAS also normalized, confirming that it was transient hyperactivity of the zona reticulata. A molecular test was performed in a blood sample by MLPA, showing a gain of methylation in the imprinting control region 1 (ICR1) of chromosome 11p15, which controls two imprinted genes, H19 and IGF-2, confirming the clinical diagnosis of BWS. The hypermethylation of ICR1 is largely related to the Wilms tumor. The patient was diagnosed with bilateral Wilms tumor at 11 months old and undergone chemotherapy without adequate response requiring left nephrectomy at 1 year and 5 days old. Conclusion: We presented the first description of false-positive diagnosis of CAH in the newborn screening of a girl with Beckwith Wiedemann syndrome, probably due to a transient overactivation of the zona reticulata. References: 1.Brioude F, Kalish JM, Mussa A, Foster AC, Bliek J, Ferrero GB, et al. Expert consensus document: clinical and molecular diagnosis, screening and management of Beckwith-Wiedemann syndrome: an international consensus statement. Nat Rev Endocrinol. 2018;14(4):229-49.