Abnormal acid gastro-oesophageal reflux is common in patients with idiopathic pulmonary fibrosis (IPF) and is considered a risk factor for development of IPF. Retrospective studies have shown improved outcomes in patients given anti-acid treatment. The aim of this study was to investigate the association between anti-acid treatment and disease progression in IPF. In an analysis of data from three randomised controlled trials, we identified patients with IPF assigned to receive placebo. Case report forms had been designed to prospectively obtain data about diagnosis and treatment of abnormal acid gastro-oesophageal reflux in each trial. The primary outcome was estimated change in forced vital capacity (FVC) at 30 weeks (mean follow-up) in patients who were and were not using a proton-pump inhibitor or histamine-receptor-2 (H2) blocker. Of the 242 patients randomly assigned to the placebo groups of the three trials, 124 (51%) were taking a proton-pump inhibitor or H2 blocker at enrolment. After adjustment for sex, baseline FVC as a percentage of predicted, and baseline diffusing capacity of the lung for carbon monoxide as a percentage of predicted, patients taking anti-acid treatment at baseline had a smaller decrease in FVC at 30 weeks (-0·06 L, 95% CI -0·11 to -0·01) than did those not taking anti-acid treatment (-0·12 L, -0·17 to -0·08; difference 0·07 L, 95% CI 0-0·14; p=0·05). Anti-acid treatment could be beneficial in patients with IPF, and abnormal acid gastro-oesophageal reflux seems to contribute to disease progression. Controlled clinical trials of anti-acid treatments are now needed. National Institutes of Health. Copyright © 2013 Elsevier Ltd. All rights reserved.