BACKGROUND An association between asthma and gastro-oesophageal reflux is well recognised but the underlying mechanism is unclear. One suggestion is that gastric juice is aspirated into the tracheal and upper airways but detection of these events is difficult and involves radioisotopic studies. A new method of making direct measurements of tracheal and oesophageal pH over a 24 hour period is described, together with its application to patients with asthma. METHODS The technique involves insertion of simultaneous tracheal and oesophageal pH probes under general anaesthesia. Continuous monitoring of pH over a 24 hour period is possible, permitting comparison with peak flow readings during wakefulness and at night should the patient be disturbed. Representative data from four patients with asthma (mean FEV1 62% predicted) and symptomatic gastro-oesophageal reflux, together with data from three non-asthmatics, is presented. RESULTS Thirty seven episodes of gastro-oesophageal reflux lasting more than five minutes were recorded. Of these, five were closely followed by a fall in tracheal pH from a mean (SE) of 7.1 (0.2) to 4.1 (0.4) and a fall in peak expiratory flow (PEFR) of 84 (16) l/min. When gastro-oesophageal reflux occurred without tracheal aspiration the fall in PEFR was 8 (4) l/min. CONCLUSIONS This new technique was well tolerated and allowed quantitation of the number, duration, and timing of episodes of tracheal micro-aspiration. Unlike acid reflux without aspiration, these events appear to be related to significant acute changes in lung function in asthmatic patients. Further studies with this new method may elucidate the role of gastro-oesophageal reflux in asthma.