The frequency of micronuclei in cells scraped from inside the human cheek is a measure of chromosome breakage in earlier cell divisions, and it can be increased tenfold by carcinogenic stimuli. Supplementation for 3 months of the diet of 40 rural Filipino betel chewers with sealed capsules of retinol (100 000 IU/week) and beta-carotene (300 000 IU/week) was associated with a threefold decrease (from 4.2% to 1.4%) in the mean proportion of cells with micronuclei. This proportion decreased in 37 of the 40 supplemented subjects and no large increases were seen in any subjects. In 11 unsupplemented betel chewers in a nearby cluster of houses the mean proportion of micronuclei did not change (4.3% before and 4.8% three months later). This suggests the possibility that in this population an increase in the dietary intake of retinol and/or carotene may reduce the incidence of oral cancer, which is an important neoplasm in many parts of Asia.