To evaluate relationships between gastroesophageal reflux (GER) and the development and onset of apparent life-threatening event(s) (ALTE), 16 infants presenting with ALTE and 6 control subjects manifesting clinical GER alone were studied using prolonged, esophageal pH monitoring in conjunction with simultaneous pulse oximetry and transthoracic impedance pneumocardiography. Despite the absence of a clinical vomiting history in 14 of 16 patients with ALTE, the incidence of GER was similar in both groups (patients with ALTE vs control subjects, 95% vs 100%). Significant arterial oxygen desaturation (less than 90% for greater than 3 minutes) was monitored during 60 episodes in 14 of 16 infants with ALTE, compared with no episodes of reduced arterial oxygen saturation in control subjects. Fifty-four of 60 of these desaturation events commenced within 3.9 +/- 0.4 minutes (mean +/- SD) of onset of a drop in esophageal pH to less than 4.0. Linear regression analysis indicates a significant correlation between duration of esophageal acidification and length of individual hypoxemic episodes (r = .39). Pneumocardiograms were normal in all patients. These data suggest that unsuspected GER is common in infants presenting with ALTE and, in these patients, GER may be directly associated with reflex hypoxemic episodes. Prolonged intraesophageal pH monitoring, performed simultaneously with evaluation for apnea, should be considered in all infants presenting with ALTE.