Sliding Type-I hiatal hernia is commonly diagnosed using upper endoscopy, barium swallow or less commonly, esophageal manometry. Current data suggest that endoscopy is superior to barium swallow or esophageal manometry. Recently, high-resolution manometry has become available for the assessment of esophageal motility. This novel technology is capable of displaying spatial and topographic pressure profiles of gastroesophageal junction and crural diaphragm in real time. The objective of the current study was to compare the specificity and sensitivity of high-resolution manometry and endoscopy in the diagnosis of sliding hiatal hernia in patients with gastroesophageal reflux disease. Data were analyzed retrospectively for 83 consecutive patients (61% females, mean age 52 ± 13.2 years) with objective gastroesophageal reflux disease who were considered for laparoscopic antireflux surgery between January 2006 and January 2009 and had preoperative high-resolution manometry and endoscopy. Manometrically, hiatal hernia was defined as separation of the gastroesophageal junction >2.0 cm from the crural diaphragm. Intraoperative diagnosis of hiatal hernia was used as the gold standard. Sensitivity, specificity and likelihood ratios of a positive test and a negative test were used to compare the performance of the two diagnostic modalities. Forty-two patients were found to have a Type-I sliding hiatal hernia (>2 cm) during surgery. Twenty-two patients had manometric criteria for a hiatal hernia by high-resolution manometry, and 36 patients were described as having a hiatal hernia by preoperative endoscopy. False positive results were significantly fewer (higher specificity) with high-resolution manometry as compared with endoscopy (4.88% vs. 31.71%, P= 0.01). There were no significant differences in the false negative results (sensitivity) between the two diagnostic modalities (47.62% vs. 45.24%, P= 0.62). Analysis of likelihood ratios of a positive and negative test demonstrated that high-resolution manometry is better than endoscopy both to rule out and rule in a hiatal hernia. A significant discordance was also observed between the two tests (P= 0.033). High-resolution manometry has better specificity and ability to rule out an overt Type-I sliding hiatal hernia (greater likelihood ratio of a positive test) in patients with GERD. Because of high false negative results, both high-resolution manometry and endoscopy are unreliable for ruling in a hiatal hernia. Negative result for a hiatal hernia by either modality mandates additional testing.