Oesophageal pH monitoring has been undertaken in 20 symptomatic patients and 20 normal subjects for periods of 24 hours at work and in the home using a pH sensitive radiotelemetry capsule and a portable receiving system. There was no correlation between symptoms and endoscopic findings in symptomatic patients. The number and duration of reflux episodes was greater in symptomatic patients than normal subjects during 24 hour ambulatory study (P less than 0.002). A frequency-duration index for the evaluation of recordings is described which was significantly greater in symptomatic patients than in normal subjects during both day (P less than 0.002) and night (P less than 0.008) periods of recording. Discrimination between the two groups was more clearly seen using the frequency-duration index compared to either the frequency of reflux episodes or the cumulative duration of acid reflux.