Potential role of beta-carotene in prevention of oral cancer. Recent data suggests that retinoids and carotenoids may be effective in reversing a putative "field cancerization" defect in the epithelium at risk for oral cancer. Animal experiments have shown that these compounds can inhibit cancer formation. Several clinical trials have demonstrated the ability of retinoids to reverse oral leukoplakia. However, toxicities associated with retinoids at the doses used in these studies limits their potential for chemoprevention. Because of its lack of toxicity, beta-carotene is a very attractive agent for chemoprevention. It suppresses micronuclei in exfoliated oral mucosal cells from subjects at risk for oral cancer and recently has been shown to be active in reversing leukoplakia. Another area under investigation is the possibility of preventing second primary tumors in patients cured of their initial cancer who have an increased risk of developing new cancers of the upper acrodigestive tract. 