Twelve hour overnight oesophageal pH monitoring in patients with reflux symptoms. Results of continuous 12 hour overnight pH monitoring (duration of pH less than 4) were reviewed in 112 patients with heartburn or regurgitation, or both, and in 56 normal subjects. Patients had more reflux than normal subjects. Medically controlled patients (n = 51) had less acid reflux than patients who subsequently underwent reflux surgery (n = 61), but there was a considerable overlap between those two groups. Surgery was followed by a reduction in acid reflux to a value similar to that in normal subjects. Patients in whom surgery was deemed to have failed had more reflux after the operation than those in whom it was successful, but no difference could be found in the preoperative reflux values of these two subgroups. Monitoring pH is not of value in selecting candidates for surgery since the results are not a good predictor of outcome, but it is useful in the objective evaluation of surgical results. 