Abstract Objectives Few data on alveolar hypoventilation in Prader‐Willi syndrome (PWS) are available and the respiratory follow‐up of these patients is not standardized. The objectives of this study were to evaluate the prevalence of alveolar hypoventilation in children with PWS and identify potential risk factors. Study Design This retrospective study included children with PWS recorded by polysomnography (PSG) with transcutaneous carbon dioxide pressure (PtcCO2) or end‐tidal CO2 (ETCO2) measurements, between 2007 and 2021, in a tertiary hospital center. The primary outcome was the presence of alveolar hypoventilation defined as partial pressure of carbon dioxide (pCO2) ≥ 50 mmHg during ≥2% of total sleep time (TST) or more than five consecutive minutes. Results Among the 57 included children (38 boys, median age 4.8 years, range 0.1–15.6, 60% treated with growth hormone [GH], 37% obese), 19 (33%) had moderate‐to‐severe obstructive sleep apnea syndrome (defined as obstructive apnea‐hypopnea index ≥5/h) and 20 (35%) had hypoventilation. The median (range) pCO2 max was 49 mmHg (38–69). Among the children with hypoventilation, 25% were asymptomatic. Median age and GH treatment were significantly higher in children with hypoventilation compared to those without. There was no significant difference in terms of sex, BMI, obstructive or central apnea‐hypopnea index between both groups. Conclusion The frequency of alveolar hypoventilation in children and adolescents with PWS is of concern and may increase with age and GH treatment. A regular screening by oximetry‐capnography appears to be indicated whatever the sex, BMI, and rate of obstructive or central apneas.