The incidence of esophageal adenocarcinoma (EA) and its precursor condition, Barrett's esophagus, has risen rapidly in the United States for reasons that are not fully understood. Therefore, we evaluated the association between use of supplemental vitamins and minerals and risk of neoplastic progression of Barrett's esophagus and EA. The Seattle Barrett's Esophagus Program is a prospective study based on 339 men and women with histologically confirmed Barrett's esophagus. Participants underwent baseline and periodic follow-up exams, which included endoscopy and self-administered questionnaires on diet, supplement use, and lifestyle characteristics. Use of multivitamins and 4 individual supplements was calculated using time-weighted averages of reported use over the observational period. Cox proportional-hazards models were used to calculate hazard ratios (HR) for each endpoint: EA, tetraploidy, and aneuploidy. During a mean follow-up of 5 yr, there were 37 cases of EA, 42 cases of tetraploidy, and 34 cases of aneuploidy. After controlling for multiple covariates including diet, nonsteroidal anti-inflammatory drug use, obesity, and smoking, participants who took 1 or more multivitamin pills/day had a significantly decreased risk of tetraploidy [HR = 0.19; 95% confidence interval (CI) = 0.08-0.47) and EA (HR = 0.38; 95% CI = 0.15-0.99] compared to those not taking multivitamins. Significant inverse associations were also observed between risk of EA and supplemental vitamin C (> or = 250 mg vs. none: HR = 0.25; 95% CI = 0.11-0.58) and vitamin E (> or = 180 mg vs. none: HR = 0.25; 95% CI = 0.10-0.60). In this cohort study, use of multivitamins and single antioxidant supplements was associated with a significantly reduced risk of EA and markers of neoplastic progression among individuals with Barrett's esophagus.