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Children with Down syndrome (DS) have a high incidence of obstructive sleep apnea (OSA) that is often associated with hypoventilation. Little is known, however, about the prevalence of sleep-associated hypoventilation independent of OSA in these children.Retrospective chart review of all children with DS under 18 years of age undergoing polysomnography at a tertiary care paediatric hospital during a 2-year period. Exclusion criteria were as follow: those requiring oxygen or positive-pressure ventilation; with tracheostomy, baseline hypoxia, unrepaired cyanotic heart disease, pulmonary hypertension, and cerebral palsy; or OSA with >5 obstructions/hour.86 children met inclusion criteria. 68 (79%) had ETCO2values >50 mm Hg during sleep. 37 (43%) ranged 50–55 mm Hg, and 12 (14%) met American Academy of Sleep Medicine criteria for hypoventilation of ETCO2 >50 mm Hg for >25% of total sleep time (TST). Average pulse-oximetry saturation (SpO2) values during sleep were 97.8% (SD ±1; range: 95.1–99.9). Average percentage of TST with SpO2 >92% was 99.89%.Mildly elevated ETCO2 in the absence of OSA is common in children with DS. This may reflect underlying differences in autonomic control of ventilation in these children and may be considered a normal variant not necessitating intervention other than close monitoring for pulmonary hypertension.