Background: The endoscopic grading of the gastroesophageal flap valve (GEFV) is suggested to be a good predictor of the reflux status. The aim of this study was to investigate the association of the GEFV and gastroesophageal and gastropharyngeal reflux. Methods: A total of 364 patients (151 men and 233 women; mean age 52.2 years) who underwent endoscopy, esophageal manometry, and ambulatory 24-hour dual probe pH monitoring were included. GEFV was graded I through IV using Hill's classification: the GEFV was largely classified into 2 groups: the normal GEFV group (grade I and II) and the abnormal GEFV group (grade III and IV). Findings of endoscopy, esophageal manometry, and ambulatory pH monitoring were compared among the groups. Results: Increased GEFV grade was significantly associated with an increased prevalence of both reflux esophagitis and Barrett's epithelium (p < 0.001). LES pressure was significantly lower in the abnormal GEFV group compared to the normal GEFV group (p < 0.001). All variables showing gastroesophageal reflux in the distal probe were significantly higher in the abnormal GEFV group compared to the normal GEFV group (p < 0.001). In addition, all variables, except the supine time of pH below 4, showing the gastropharyngeal reflux in the proximal probe were significantly higher in the abnormal GEFV group compared to the normal GEFV group (p < 0.001). The frequency of gastroesophageal reflux disease and gastropharyngeal reflux disease was significantly higher in the abnormal GEFV group compared to the normal GEFV group (p < 0.001). Conclusions: Endoscopic grading of the GEFV is easy and provides useful information about the status of gastroesophageal and gastrophryngeal reflux.