The field cancerization hypothesis suggests that carcinogen exposure affects the entire epithelial lining of the lungs and upper aerodigestive tract. The concept that common exposures place the entire epithelium at risk for the development of invasive cancer is supported both by the occurrence of premalignant lesions such as leukoplakia and squamous metaplasia, and by the development of multiple primary tumors within the field. Chemoprevention trials in lung and upper aerodigestive tract cancer have included studies to reverse premalignant lesions and to prevent second primary tumors. Promising results have been reported in both settings using the retinoid 13-cis-retinoid acid. Several clinical trials are in progress which attempt both to reduce cancer incidence and to determine the mechanisms and biological markers of successful chemoprevention.