Milk thickening agents are believed to reduce episodes of gastroesophageal reflux, but their use has not been evaluated thoroughly. We studied the effect of these agents in 30 bottle-fed babies, 6-8 weeks old, with clinical gastroesophageal reflux pathology. Continuous 24-hour esophageal pH monitoring revealed gastroesophageal reflux pathology for all parameters studied: reflux index (18.4%), duration of the longest reflux episode (23.3 min), number of reflux episodes in 24 h (34.5), number of reflux episodes greater than 5 min (6.8). All investigations were performed in prone-anti-Trendelenburg position. The infants were treated with milk thickening agents (1 g to 115 ml, as recommended by Carre). Most (n = 25) showed clinical improvement of their symptoms. A second pH monitoring was performed under treatment conditions after 7-14 days, and showed in 24 infants a decrease of the number of reflux episodes (15.1 in 24 h) (p less than 0.001), but a comparable reflux index (17.8%) (NS) and number of long lasting (greater than 5 min) reflux episodes (7.8) (NS). The duration of the longest reflux episode, however, increased significantly (56.6 min) (p less than 0.001). In six infants all parameters were within normal ranges at follow-up. Milk thickening agents seem clinically effective as a treatment for gastroesophageal reflux in individual cases, but can lead to occult gastroesophageal reflux episodes of long duration, possibly increasing the risk for esophagitis or respiratory dysfunction.