A control group of infants was evaluated to determine criteria for the diagnosis of histologic esophagitis. Based on our observations, histologic esophagitis was defined as four or more intraepithelial neutrophils or one eosinophil per high power field or both. Esophageal biopsy specimens from 33 consecutive infants younger than 2 years who had been examined for clinically significant gastroesophageal reflux (GER) were reviewed for histologic esophagitis. Endoscopy had been performed in each patient, and 4.1±1.1 (mean±SD) biopsy specimens had been obtained above the distal 20% of the esophagus. Twenty (61%) infants had histologic esophagitis, including 15 with intraepithelial eosinophils alone, one with intraepithelial neutrophils alone, and four with both. Older infants (7 to 24 months) with histologic esophagitis were more likely to have moderate to severe inflammation than were infants younger than 7 months of age (P=0.01). Endoscopic evidence for gross esophagitis was found in six (18%) infants; of these, five had abnormal biopsies, including four with moderate to severe inflammation. Among the 27 infants with a grossly normal esophagus, 14 (52%) had histologic esophagitis, including nine (33%) with moderate to severe inflammation. We conclude that in infants with clinically significant GER: (1) esophagitis is common, (2) histologic esophagitis frequently occurs in the absence of gross endoscopic findings, (3) the likelihood of moderate to severe inflammation increases after 6 months of age, and (4) intraepithelial eosinophils are a sensitive marker for acute inflammation in association with GER. © 1985 The C. V. Mosby Company.