['Some patients with eosinophilic esophagitis (EoE) have an extremely narrow esophagus, but the characteristics of this group have not been extensively described. We aimed to characterize the narrow-caliber phenotype of EoE, determine associated risk factors, and identify differences in treatment response in this subgroup of patients.', 'This retrospective cohort study from 2001 to 2014 included subjects with a new diagnosis of EoE per consensus guidelines. Demographic, endoscopic, histologic, and treatment response data were extracted from medical records. An extremely narrow-caliber esophagus was defined when a neonatal endoscope was required to traverse the esophagus due to the inability to pass an adult endoscope. Patients with and without an extremely narrow-caliber esophagus were compared. Multivariable logistical regression was performed to assess treatment outcomes.', 'Of 513 patients with EoE, 46 (9%) had an extremely narrow-caliber esophagus. These patients were older (33 vs 22 years; P\xa0< .01), had longer symptom duration (11 vs 3 years; P\xa0< .01), more dysphagia (98% vs 66%; P\xa0< .01), and food impactions (53% vs 31%; P\xa0< .01). Dilation was more common with extreme narrowing (69% vs 17%; P\xa0< .01). Patients with a narrow-caliber esophagus were more refractory to steroid treatment, with lower symptom (56% vs 85%), endoscopic (52% vs 76%), and histologic (33% vs 63%) responses (P\xa0< .01 for all), and these differences persisted after multivariate analysis.'] Is the extremely narrow-caliber esophagus a treatment-resistant subphenotype of eosinophilic esophagitis?