The development of lung cancer is associated with aberrant promoter methylation and thus transcriptional silencing of many tumor suppressor genes or genes critical for cellular maintenance. Here we report that the NADPH oxidases DUOX1 and DUOX2, which are one of the main sources for reactive oxygen species production in the airway, are frequently silenced in human lung cancer. Screening of lung cancer cell lines revealed loss of DUOX1 and DUOX2 expression, which was restored after treatment with 5-aza 2'-deoxycytidine. Two genes, DUOXA1 and DUOXA2, which are transcriptionally and functionally linked to DUOX, also showed coordinated down-regulation in lung cancer cells and lung cancer specimen. Bisulfite sequencing and methylation-specific PCR revealed that CpG-rich promoter regions in both DUOX genes are hypermethylated. Epigenetic modification of at least one DUOX gene was detected in 50% of primary adenocarcinomas. Immunohistochemical analysis of airway sections derived from cancerous and matched healthy tissues confirmed down-regulation of Duox in the ciliated epithelial cells lining the respiratory tract. Reintroduction of functional Duox1 into lung cancer cell lines increased cell migration and wound repair without affecting cell growth. Our results suggest that an area on chromosome 15 that includes DUOX1, DUOX2, and their maturation factors is a frequent target for epigenetic silencing in lung cancer.