BACKGROUND & AIMS: Familial clustering of eosinophilic esophagitis (EoE) has been described, and we report on the biopsy-assessed prevalence of esophageal eosinophilia (EE) in first-degree family members. The aim was to determine the prevalence of EE in first-degree adult relatives (FDRs) of EoE patients. METHODS: Index EoE patients diagnosed by EE (>15 eosinophils per high-power field) and proton pump inhibitor nonresponsiveness were identified and family trees were constructed. Adult FDRs were invited to undergo upper endoscopy with esophageal biopsies and to complete reflux, dysphagia, and allergy/atopy questionnaires. Questionnaire information was gathered only for those who responded as per institutional review board purview. Records from other children and adult FDRs with prior EoE diagnoses also were obtained when permission was obtained. Simple and multivariable logistic regression models were used to evaluate the unadjusted and odds ratios of EoE for demographic and clinical variables. RESULTS: A total of 239 FDRs from 37 index EoE patients were identified. Seventy-one of 239 adult (age, >18 y) FDRs completed endoscopy and questionnaires and 18 of 71 FDRs had EE. An additional 17 FDRs were confirmed to have EE after external medical record retrieval, resulting in a total of 35 of 239 (14.6%) FDRs with EE. Significantly more male FDRs had EE compared with female FDRs (P = .027). Proton pump inhibitors, dysphagia, gastroesophageal reflux disease, asthma, and reflux symptoms predicted EE in FDRs. FDRs who had EE reported hay fever, allergic eye symptoms, and food allergy more frequently than those without EE (P = .03, P = .001, and P = .02, respectively). Specifically, younger age, higher serum eosinophils, being male, and having food allergies all were associated with higher odds of EoE (P = .0211, P = .0031, P = .0362, and P = .0089, respectively). CONCLUSIONS: The prevalence of esophageal eosinophilia is extremely high and male-predominant in first-degree relatives of EoE patients. Symptoms of hay fever, allergic eye symptoms, and food allergy were predictors of EE in FDRs. Dysphagia did not predict esophageal eosinophilia. Family members of EoE patients are at risk for EE, particularly those who have atopic symptoms.