BACKGROUND: Chronic oesophageal inflammation and related oxidative stress are important in the pathogenesis of erosive oesophagitis (EO) and its malignant progression. AIM: To study the effect of proton pump inhibitors (PPIs) on oesophageal cellular immune response and oxidative damage in EO patients. METHODS: Forty gastro-oesophageal reflux disease (GERD) patients [non-erosive reflux disease (NERD): 15, EO: 25] were included, after 7 days off antisuppressive drugs. EO patients were randomized to 20-mg rabeprazole once daily for either 4 or 8 weeks with baseline and follow-up endoscopy with distal oesophageal biopsies. T lymphocytes, macrophages and mast cells were quantified by immunohistochemistry. DNA adducts were measured by analysis of 8-oxo-deoxyguanosine levels. RESULTS: Erosive oesophagitis patients had more T lymphocytes and CD8(+) T lymphocytes in squamous epithelium than NERD patients (P = 0.001, P = 0.002, respectively). Levels of DNA adducts between both groups were, however, not different (P = 0.99). Four- and eight-week rabeprazole treatment in EO patients resulted in a significant decrease in number of T lymphocytes and CD8(+) T lymphocytes (all P < 0.05). PPIs did not, however, affect levels of DNA adducts. CONCLUSIONS: Short-term PPI therapy in EO patients reduces the oesophageal cellular immune response, but does not change oxidative damage. PPI therapy may therefore not be effective in reducing the risk of oesophageal cancer in GERD patients.