The significance of marked eosinophilic infiltration in esophageal mucosal biopsy specimens was evaluated in 11 patients. The patients were generally young, with an average age of 14.6 years; all had diffuse intraepithelial eosinophilia in several biopsies. Ten patients (91%) had evidence for reflux esophagitis, which was associated with esophageal stricture in three of the six patients older than 1 year. Marked esophageal eosinophilia might therefore indicate prolonged or severe gastroesophageal reflux. One patient with peripheral eosinophilia, a history of asthma, and concurrent idiopathic eosinophilic gastroenteritis lacked evidence of reflux and represents a case of idiopathic esophagitis. Critical review of the literature establishes three additional cases. Idiopathic eosinophilic esophagitis is an unusual variant of idiopathic, but presumably allergic, eosinophilic infiltration of the gastrointestinal tract.