Forty per cent of patients with heartburn may have no evidence of oesophagitis or of abnormal oesophageal acid exposure. The symptom index correlates symptoms that occur during pH monitoring with episodes of acid reflux, being the number of symptoms during reflux divided by the total number occurring during monitoring. This index was assessed in 61 patients with heartburn. In 39 patients with endoscopic evidence of oesophagitis or abnormal acid exposure times on pH monitoring, the symptom index had a sensitivity of 90 per cent. Of the 22 patients with no objective abnormality, the index was > or = 50 per cent in five (23 per cent) whose symptoms were presumably due to acid reflux despite results of other investigations being normal, suggesting an acid-sensitive oesophagus. The symptom index is a useful additional measure in the investigation of patients with suspected gastro-oesophageal reflux disease.