Gastro-oesophageal reflux (GOR) has been implicated in the aetiology of lung disease. Cystic fibrosis (CF) patients have a high incidence of GOR symptoms with demonstrable episodes of oesophageal acidification. We studied 24-hour ambulatory tracheal and oesophageal pH in 11 CF patients with GOR symptoms to identify any episodes of tracheal acidification and define their temporal relation to oesophageal reflux and respiratory symptoms. 8 patients had evidence of significant GOR (DeMeester score mean 58; range 17-107) and in 6 it was gross (DeMeester score > 30). 4 patients had tracheal acidification (defined as tracheal pH < 5.5): all had greatly raised DeMeester scores. Two patterns of lowered tracheal pH were seen: a gradual drift downwards of tracheal pH to < 5.5 which recovered slowly, and an acute fall in tracheal pH to < 5.5 with rapid recovery. Only one patient had a fall in peak expiratory flow in conjunction with a decline in tracheal pH, and no association was found between the presence of tracheal microaspiration and pulmonary function. We conclude that tracheal acidification occurs in adult CF patients with GOR.