Although children commonly travel to high altitudes, their respiratory adaptation to hypoxia remains elusive. Therefore, in the present study respiratory inductive plethysmography, pulse oximetry (S(p,O(2))) and end-tidal CO(2) tension (P(ET,CO(2))) were recorded in 20 pre-pubertal children (aged 9-12 yrs) and their fathers during 1 night in Zurich (490 m) and 2 nights at the Swiss Jungfrau-Joch research station (3,450 m) following ascent by train within <3 h. In children, mean+/-sd nocturnal S(p,O(2)) fell from 98+/-1% at 490 m to 85+/-4 and 86+/-4% at 3,450 m (nights 1 and 2, respectively); P(ET,CO(2)) decreased significantly from 37+/-6 to 32+/-3 and 33+/-4 mmHg (3,450 versus 490 m). In adults, changes in nocturnal S(p,O(2)) and P(ET,CO(2)) at 3,450 m were similar to those in children. Children spent less time in periodic breathing at 3,450 m during night 1 and 2 (8+/-11 and 9+/-13%, respectively) than adults (34+/-24 and 22+/-17%, respectively), and their apnoea threshold for CO(2) was lower compared with adults (27+/-2 and 30+/-2 mmHg, respectively, both nights). S(p,O(2)), P(ET,CO(2)) and time in periodic breathing at altitude were not correlated between children and their fathers. In conclusion, children revealed a similarly reduced nocturnal O(2) saturation and associated hyperventilation at high altitude as adults but their breathing pattern was more stable, possibly related to a lower apnoea threshold for CO(2).