Introduction:Multiple surgical methods exist to restore the antireflux mechanism of the lower esophageal sphincter (LES). Techniques can be grouped into 3 categories: fundoplication, reconstruction of the angle of His and esophago-gastropexy and reconstruction of the valve mechanism. In a randomized controlled trial comparing the Hill repair with the Nissen fundoplication, we hypothesized there would be manometric differences between the 2 procedures that could explain their respective mechanisms of reflux prevention. Methods: 123 patients (Hill = 52, Nissen = 71) were randomized under an IRB protocol to receive either a laparoscopic Nissen or Hill procedure. To avoid institutional procedure bias, all procedures were done under the guidance of a Nissen (LS) or Hill (RA) expert. Standard preoperative and post operative testing were performed. For this study, high resolution manometry performed after surgery was available for 32 patients. High resolution manometry was analyzed and interpreted by blinded observers marking the following results: proximal and distal LES borders, maximum and nadir LES pressure, esophageal body peristalsis, percent relaxation, and distal amplitude. Results:32 patients (15 Hill, 17 Nissen) underwent post operative high resolution manometry (mean = 15 months). All patients had normal post-operative overall LES length (3.7 cm Hill vs. 3.3 cm Nissen, p=0.9). More patients had an increase in the LES length in the Nissen group compared to pre-op (0.9 cm Nissen vs -0.5 cm Hill, p=0.02). However, no difference was seen in intra-abdominal length between the Hill and Nissen (2.0 cm vs. 2.1 cm, p=0.9), resting pressure (19.4 mmHg vs. 20.4 mmHg, p=0.8), relaxation (residual pressure 10 mmHg vs. 12 mmHg, p=0.5), esophageal body function (peristalsis 92% vs. 89%, p=0.6), or distal esophageal contraction amplitude (92mmHg vs. 89 mmHg, p=0.8). A short intra-abdominal length (< 1.49 cm) was found in 2/15 Hill and 5/17 Nissen patients (p=0.3). Conclusion:Overall, the Hill repair and the Nissen fundoplication reconstruct the LES with similar characteristics. There was a non-significant trend toward restoring more LES length in the Nissen but the Hill may be better at restoring intra-abdominal length. These findings correspond to the differences in surgical technique and may indicate a role for the Hill in patients with shortened esophagus.