Background & Aims Acid exposure contributes to the development of Barrett's esophagus (BE) and its progression toward esophageal adenocarcinoma. Patients with BE are frequently treated with acid suppressants, but it is unclear whether these prevent the development of BE-related cancer. We investigated whether acid suppression reduces the risk of neoplastic progression in patients with BE. Methods We performed a multicenter prospective cohort study of 540 patients with BE. We collected information on medication use at each surveillance visit, which was cross-checked with pharmacy records. Patients also completed a questionnaire about their use of over-the-counter medication. Incident cases of high-grade dysplasia and esophageal adenocarcinoma were identified during a median follow-up period of 5.2 years. Time-dependent Cox regression models were used to investigate the effect of acid suppression on the risk of neoplastic progression. Results Forty patients (7%) developed high-grade dysplasia or esophageal adenocarcinoma during the follow-up period. Use of histamine-2 receptor antagonists did not affect the incidence of neoplastic progression. However, use of proton pump inhibitors (PPIs) at inclusion in the study or during the follow-up period reduced the risk of neoplastic progression (hazard ratio, 0.41; 95% confidence interval, 0.18–0.93 and hazard ratio, 0.21; 95% confidence interval, 0.07–0.66). Prolonged use of PPIs and good adherence were associated with an additional protective effect. The prevalence of esophagitis decreased during PPI use, but length of BE was not affected. Conclusions In a multicenter prospective cohort study, PPI use was associated with a reduced risk of neoplastic progression in patients with BE.