Background: The prevalence of gastroesophageal reflux disease (GERD) has reportedly risen in recent years. Difficulties associated with endoscopic diagnosis mean it is not easy to determine its precise prevalence. A prospective study of the prevalence of endoscopy-positive GERD (EP-GERD) was conducted at Higuchi Hospital, a general hospital in Northwestern Kyushu, Japan. The study also correlated factors that might affect prevalence (age, sex, and functions of the gastroesophageal junction). Methods: From consecutive patients undergoing endoscopic examination at Higuchi Hospital between January 2000 and April 2003, 1234 patients without severe complications were examined for the possible presence of GERD. Patients were stratified by age and sex, and the prevalence in each group ascertained. EP-GERD was defined on the four-level scale of the Los Angeles classification. Endoscopic classification of gastroesophageal flap valve ([GEFV] functional anomalie; using a four-level scale), was done as proposed by Hill et al. in 1996, to assess flap-valve morphology. Six items were evaluated: (1) symptoms and primary diseases; (2) prevalence of development of EP-GERD classified by age and sex; (3) endoscopic morphology of the GEFV as an expression of the functions of the gastroesophageal junction, and its prevalence by age and sex; (4) regression analysis and Spearman’s rank correlation of GEFV and EP-GERD grades; (5) prevalence of EP-GERD and GEFV stratified by age and analyzed; and (6) multiple regression analysis of EP-GERD and explanatory variables (age, sex, and GEFV). Results: The overall prevalence of EP-GERD was 5.8%(72/1234) and this patient group was dominated by men. Aging had minimal effect on prevalence in men, but the prevalence rose among women as they aged. The age-stratified prevalence of GEFV functional anomalies was similar to the age-stratified prevalence of EP-GERD in both sexes. The correlation between EP-GERD and GEFV functional anomalies was high regardless of sex. Conclusions: We postulate that the mechanisms leading to the development of GEFV functional anomalies in men are different from those in women. Future evaluations of EP-GERD should also observe GEFV function. Background: The prevalence of gastroesophageal reflux disease (GERD) has reportedly risen in recent years. A prospective study of the prevalence of endoscopy-positive GERD (EP-GERD) was conducted at Higuchi Hospital, a general hospital in Northwestern Kyushu, Japan. The study also correlated factors that might affect prevalence (age, sex, and functions of the gastroesophageal junction). Methods: From consecutive patients undergoing endoscopic examination at Higuchi Hospital between January 2000 and April 2003, 1234 patients without severe complications were examined for the possible presence of GERD. Patients were stratified by age and sex, and the prevalence in each group ascertained. EP-GERD was defined on the four-level scale of the Los Angeles classification. Endoscopic classification of gastroesophageal flap valve ([GEFV] functional anomalie; using a four-level scale), was do ne as proposed by Hill et al. in 1996, to assess flap-valve morphology. Six items were: (1) symptoms and primary diseases; (2) prevalence of development of EP-GERD classified by age and sex; endoscopic morphology of the GEFV as an expression of the functions of the gastroesophageal junction, and its prevalence by age and sex; (4) regression analysis and Spearman’s rank correlation of GEFV and EP-GERD grades; (5) prevalence of EP-GERD and GEFV stratified by age and analyzed; and (6) multiple regression analysis of EP-GERD and explanatory variables (age, sex, and GEFV). Results: The overall prevalence of EP-GERD was 5.8% (72/1234) and this patient Aging had minimal effect on prevalence in men, but the prevalence rose among women as they aged. The age-stratified prevalence of GEFV functional anomalies were similar to the age-stratified prevalence of EP-GERD in both sexes. The correlation between EP-GERD and GEFV functional anomalies was high regardless of sex. Conclusions: We postulate that the mechanisms leading to the development of GEFV functional anomalies in men are different from those in women. Future evaluations of EP-GERD should also observe the GEFV function.