This intervention trial carried out in Uzbekistan (former USSR) in an area with a high incidence of oral and esophageal cancer involved random allocation of 532 men, 50 to 69 years old, with oral leukoplakia and/or chronic esophagitis to one of four arms in a double-blind, two-by-two factorial design, with active arms defined by the administration of (a) riboflavin; (b) a combination of retinol, beta-carotene, and vitamin E; or (c) both. Weekly doses were 100,000 IU of retinol, 80 mg of vitamin E, and 80 mg of riboflavin. The dose of beta-carotene was 40 mg/d. Men in the trial were followed for 20 months after randomization. The aim of the trial was to determine whether treatment with these vitamins or their combination could affect the prevalence of oral leukoplakia and/or protect against progression of oral leukoplakia and esophagitis, conditions considered to be precursors of cancer of the mouth and esophagus. A significant decrease in the prevalence odds ratio (OR) of oral leukoplakia was observed after 6 months of treatment in men receiving retinol, beta-carotene, and vitamin E (OR = 0.62; 95% confidence interval (CI): 0.39 to 0.98). After 20 months of treatment, no effect of vitamin supplementation was seen when the changes in chronic esophagitis were compared in the four different treatment groups, although the risk of progression of chronic esophagitis was lower in the subjects allocated to receive retinol, beta-carotene and vitamin E (OR = 0.65; 95% CI: 0.29 to 1.48) A secondary analysis not based on the randomized design revealed a decrease in the prevalence of oral leukoplakia in men with medium (OR = 0.45; 95% CI: 0.21 to 0.96) and high (OR = 0.59; 95% CI: 0.29 to 1.20) blood concentrations of beta-carotene after 20 months of treatment. Risk of progression of chronic esophagitis was also lower in men with a high blood concentration of beta-carotene, odds ratios being 0.30 (95% CI: 0.10 to 0.89) and 0.49 (95% CI: 0.15 to 1.58) for medium and high levels, respectively. A decrease in risk, also statistically not significant, was observed for high vitamin E levels (OR = 0.39; 95% CI: 0.14 to 1.10). These results were based on levels of vitamins in blood drawn after 20 months of treatment.