The frequency and duration of gastro-oesophageal reflux were examined in 40 preterm infants and compared with a previously published healthy cohort of 74 term infants. Selection required that the infants were born between 24 and 32 weeks' gestation, had a normal head ultrasound and were studied at term post-menstrual age. Multi-channel pen recordings of sleep state, movement, breathing and acid reflux were made. In term and preterm infants the frequency and duration of reflux were greatest in active sleep, rare in quiet sleep and significantly less in preterm than term infants in wakefulness and active sleep (p < 0.05). The fewer and shorter episodes in preterm infants could not be explained by sleep state movement, gestational or postnatal age, days intubated, days on oxygen or the lowest pH of reflux episodes.