BACKGROUND: Hiatal hernia is diagnosed by barium-swallow esophagogram or esophagogastroduodenoscopy, with possible suboptimal results. High-resolution manometry clearly identifies crural diaphragm and lower esophageal sphincter. OBJECTIVES: To assess the diagnostic accuracy of high-resolution manometry in detecting hiatal hernia compared to esophagogram and esophagogastroduodenoscopy, using as reference the surgical in vivo measurement. METHODS: Patients were studied with esophagogram, esophagogastroduodenoscopy, high-resolution manometry and in vivo evaluation of the esophago-gastric junction. Esophago-gastric junction was classified as type I (no separation between crural diaphragm and lower esophageal sphincter); type II (≥1, ≤ 2 cm separation); type III (>2 cm). During in vivo measurement, distance between the esophago-gastric junction and crural diaphragm proximal border was recorded. RESULTS: Surgery identified 53 hiatal hernias in 100 patients. Forty-seven percent were classified as type I esophago-gastric junction, 35% type II and 18% type III. Referenced to in vivo evaluation, high-resolution manometry showed superior diagnostic sensitivity and specificity (94.3% and 91.5%, respectively) to esophagogram and esophagogastroduodenoscopy, with 92.6% predictive value of a positive test and 93.5% predictive value of a negative test. The kappa value for high-resolution manometry and in vivo evaluation was 0.85. High-resolution manometry showed optimal sensitivity and specificity in detecting types I, II and III esophago-gastric junction. CONCLUSIONS: High-resolution manometry enables an accurate diagnosis of hiatal hernia and a better classification than endoscopy and radiology, reaching optimal agreement with in vivo assessment.