Alterations in global DNA methylation have been observed in many cancers, but whether such alterations represent an epigenetic difference in susceptibility for the disease is unknown. The status of global DNA methylation also has not been reported in intact or specific types of cells involved in the carcinogenic process. To address these issues in lung carcinogenesis, we evaluated the status of global DNA methylation by using a monoclonal antibody specific for 5-methylcytosine (5-mc) in randomly selected lung specimens of 60 cigarette smokers who developed squamous cell carcinoma (SCC) and 30 cigarette smokers who did not. 5-mc immunostaining scores of DNA of SCC (0.61 ± 0.42) and associated hyperplastic lesions (0.82 ± 0.27) was significantly lower than those of DNA of histologically normal bronchial epithelial cells (0.99 ± 0.52) and hyperplastic lesions (1.2 ± 0.22) of noncancer specimens. The ratio of 5-mc scores between SCC and matched uninvolved bronchial epithelial cells was significantly associated with advanced stage and size of the tumor. The results suggest that alteration in global DNA methylation is an important epigenetic difference in susceptibility for the development of lung cancer. The reduced global DNA methylation in SCC compared with epithelial hyperplasia and its association with tumor size and disease stage is suggestive of its involvement in the progression of SCC. The results also indicate that normal methylation of DNA in epithelial hyperplastic lesions may prevent the transformation of these lesions to invasive cancer. If these results are confirmed, the status of DNA methylation in early lesions such as epithelial hyperplasia could be used to identify smokers who are at risk for the development of SCC. Copyright © 2001 by W.B. Saunders Company.