Barrett's oesophagus, a significant complication of gastro-oesophageal reflux disease (GERD), is the single most important risk factor for oesophageal adenocarcinoma. The strong association between Barrett's oesophagus and chronic GERD suggests that abnormal oesophageal acid exposure plays an important role in this condition. The progression of Barrett's oesophagus from specialized intestinal metaplasia to dysplasia and finally invasive carcinoma is incompletely understood, but increased and disordered proliferation is a key cellular event. In ex vivo organ culture experiments, cell proliferation is increased after exposure to short pulses of acid, whilst proliferation is reduced in Barrett's oesophagus specimens taken from patients with oesophageal acid exposure normalized by antisecretory therapy. In long-term clinical studies, consistent and profound intra-oesophageal acid suppression with proton pump inhibitors decreases cell proliferation and increases differentiation in Barrett's oesophagus, but the clinical importance of such favourable effects on these surrogate markers is not clear. In clinical practice, proton pump inhibitors relieve symptoms and induce partial regression to squamous epithelium, but abnormal oesophageal acid exposure and the risk for dysplasia or adenocarcinoma persist in many patients. The ability of proton pump inhibitors to suppress acid profoundly and consistently may be critical in the long-term management of Barrett's oesophagus.