In a group of 60 patients with symptomatic gastroesophageal reflux (GER), we carried out upper gastrointestinal (GI) endoscopy and 24-h ambulatory esophageal pH monitoring to assess the relationship between acid reflux and esophagitis. The results of 24-h pH measurement were compared with those of 15 asymptomatic control subjects who were studied with ambulatory 24-h esophageal pH monitoring only. Thirty-two patients (53.3%) had a normal esophagus macroscopically, and 28 patients (46.7%) had some degree of esophagitis. There was no significant difference between the two groups with and without esophagitis, regarding male:female ratio, age, and duration of symptoms. The group with esophagitis was more symptomatic (p less than 0.001) than the group without, and differed significantly in relation to all pH variables, i.e., number of GER episodes per hour, duration of mucosal exposure to acid (pH less than 4), and number of GER episodes requiring more than 5 min to clear per hour for the upright, supine, and 24-h periods, compared with the control group (p less than 0.001) and the group without esophagitis (p less than 0.001). In the group with esophagitis, comparison of the above pH variables in the upright and supine periods showed significantly higher values in the upright than in the supine period for the total number of reflux episodes per hour (p less than 0.001) and the number of episodes greater than 5 min/h (p less than 0.05). We conclude that the presence of esophagitis is related to both frequency and duration of GER episodes. Our findings also stress the importance of daytime acid exposure in the pathogenesis of esophagitis.