We investigated whether acid reflux in the proximal esophagus can induce arousal from sleep in infants. Fifty normal infants with occasional regurgitations were studied at the age of 8 weeks (range 4-26 weeks). In each child a pH probe was placed in the proximal portion of the thoracic esophagus, in front of the third vertebra, under radiologic control. Polygraphic monitoring of state of alertness and of proximal esophageal pH changes was recorded continuously during one night. The data were analyzed blind. For 41 of the 50 infants, a total of 97 drops below pH 4 was computed. Reflux was more frequently associated with wakefulness (41% of the episodes), or rapid eye movement (REM) sleep (39%), than with nonREM (19%) or indeterminate sleep (1%). Comparing the fifth and the last minutes preceding the pH drops, a significant increase in the number of behavioral arousals was observed (p = 0.003). In comparison with the minute before the drop in esophageal pH, a further significant increase in the number of arousals occurred during the first minutes following the pH drop (p = 0.001). Although the first minutes following the 97 episodes of reflux represented only 0.4% of the total sleep time, 76% of all arousals (74 out of 97) took place during this time period (p = 0.001). Five minutes after the pH drops, the behavioral changes tended to return to prereflux values. It is concluded that in infants, during sleep, proximal gastroesophageal reflux can act as a strong arousal stimulus.