OBJECTIVE: A recent study evidenced by metyrapone test a central adrenal insufficiency (CAI) in 60% of Prader–Willi syndrome (PWS) children. These results were not confirmed in investigations with low [Low-Dose Tetracosactrin Stimulation Test (LDTST), 1 μg] or standard-dose tetracosactrin stimulation tests. We extended the research by LDTST in paediatric patients with PWS. DESIGN: Cross-sectional evaluation of adrenal stress response to LDTST in a PWS cohort of a tertiary care referral centre. PATIENTS: Eighty-four children with PWS. MEASUREMENTS: Assessment of adrenal response by morning cortisol and ACTH dosage, and 1-μg tetracosactrin test. Response was considered appropriate when cortisol reached 500 nm; below this threshold, patients were submitted to a second test. Responses were correlated with the patientsʼ clinical and molecular characteristics to assess genotype–phenotype correlation. RESULTS: Pathological cortisol peak responses to the LDTST were registered in 12 patients (14·3%) who had reduced basal (169·4 ± 83·3 nm) and stimulated (428·1 ± 69·6 nm) cortisol levels compared to patients with normal responses (367·1 ± 170·6 and 775·9 ± 191·3 nm, P<0·001). Body mass index standard deviation score was negatively correlated with basal and peak cortisol levels (both P < 0·001), and the patientsʼ ages (P<0·001). In patients with deletion on chromosome 15, the cortisol peak was significantly lower than that in uniparental disomy (UPD) cases (P=0·030). At multiple regression analysis, the predictors of peak response were basal cortisol, age, and UPD subclass (r = 0·353, P<0·001). Standard-dose (250 μg) tetracosactrin test confirmed CAI in 4/12 patients (4·8% of the cohort). CONCLUSIONS: Our results support the hypothesis that, albeit rare, CAI may be part of the PWS in childhood.