In a clinical trial the effect of chemoprevention with beta-carotene, vitamin E and C on dysplastic tissue was studied. The study included 24 patients with oral leukoplakia and 24 patients after radical resection of a primary oral cancer. There was a reduction of increased cell kinetic parameters like the S-phase portion or the average number of nuclear-organizer regions (NOR) per cell nucleus, a decrease of the micronuclei portion and a normalization of the cytokeratin gene-expression. The general response was 97.5%. Stopping the alcohol and tobacco abuse the effect of the antioxidative vitamins on redifferentiation of the oral mucosa was more intense than by persistance of the alcohol and tobacco abuse, but a long term prevention seems to be ineffective.