The effect of two different types of feedings on results of esophageal pH monitoring was prospectively studied in 49 infants undergoing evaluation for gastroesophageal reflux (GER). Infants were randomly assigned to receive either apple juice (AJ) or formula for the first feeding during extended pH monitoring (EPM). Each infant received the alternate liquid for the second feeding. During the rest of the monitoring period, infants received formula feedings. The percentage of time that esophageal pH was less than 4.0 following both types of feedings and the percentage of time that pH was lower than 5.0 following formula feedings were determined. Following AJ feeding, the mean percentage of time pH was less than 4.0 was 43.8% in contrast to 5.1% following formula feeding. However, following formula feeding, pH was less than 5.0 35.7% of time, similar to the percentage of time pH was less than 4.0 after AJ feeding. Ability to detect GER with short-term monitoring after the two feedings was compared to detection following extended monitoring. Detection of GER with short-term monitoring following AJ feeding correlated well with extended monitoring (r = 0.67; p less than 0.001). There was a weaker, although significant, correlation between short-term monitoring following formula feeding using pH less than 5.0 as the reflux criterion and extended monitoring (r = 0.3; p less than 0.01). We conclude that outpatient GER evaluation with intraesophageal pH monitoring during a feeding interval following an AJ feeding may serve as an acceptable substitute for extended pH monitoring when it is not practical or desirable to admit the patient to the hospital.