BACKGROUND/AIMS: In the preterm infants, gastroesophageal reflux (GER) has been associated with apnea. The aim of this study was to determine the effects of feeding, chest physiotherapy, and me chanical ventilation on GER, and to evaluate the association between GER and apnea in preterm infants. METHODS: The 24 hour esophageal pH was recorded using a monocrystalline antimony electrode for 22 preterm infants. The mean gestational age and postconceptional age were 31 and 34 weeks, respectively. The mean birth weight was 1,835 g. Patients were classified into three groups eight infants without apnea; seven infants with xanthine sensitive apnea; seven infants with xanthine resistant apnea who had frequent attacks of apnea that were unaffected by the adminstration of ami nophylline. RESULTS: The mean reflux index was 7.2% and the longest episode was 17.8 minutes Physiotherapy was associated with a significant increase in the amount of reflux. The GER index during mechanical ventilation (1.0%) was significantly less than that during normal breathing (9.0%) The mean reflux index in no apnea group, xanthine sensitive apnea group, and xanthine resistan apnea group was 3.9%, 5.6%, and 12.6%, respectively. CONCLUSIONS: GER should be considered as a cause of xanthine resistant apnea in preterm infants.