Since a high concentration of beta-carotene in blood reduces the risk of lung cancer, a large-scale intervention examination containing beta-carotene was conducted, mainly by the National Cancer Institute. The results showed that the risk of lung cancer increased with administration of beta-carotene. This result demonstrates that continuation of smoking is an important factor in the increased risk, and not smoking is confirmed to be the most important prevention method. The authors examined the treatment effect of raising the concentration of folic acid and vitamin B12 in blood on bronchial dysplasia as a pre-cancerous lesion. A significant medical treatment effect was see in the folic acid and vitamin B12 medication groups, which seems promising for the chemoprevention of lung cancer.