Abstract Purpose: CYP1B1 and CYP1A1 expression is up-regulated by activation of the aryl hydrocarbon receptor (AhR) through binding of ligands such as cigarette smoke components. We examined the association between AhR, CYP1B1, and CYP1A1 expression in noninvasive bronchioloalveolar carcinomas (BAC) and lung adenocarcinomas and investigated the effects of AhR overexpression on cell physiology. Experimental Design: AhR, CYP1B1, and CYP1A1 expression was examined in 107 lung adenocarcinomas and 57 BAC by immunohistochemistry. AhR expression in lung adenocarcinoma H1355 cells was stably reduced by RNA interference (RNAi). AhR, CYP1B1, and CYP1A1 expression was examined using real-time reverse transcription-PCR. Cell physiology was evaluated by measuring anchorage-independent growth and intracellular reactive oxygen species. Results: Expression of AhR and CYP1A1 was associated in smoking adenocarcinoma patients, whereas expression of AhR and CYP1B1 was associated regardless of smoking status. The level of CYP1B1, but not CYP1A1, was positively associated with AhR overexpression in BAC. 2,3,7,8-Tetrachlorobenzo-p-dioxin–induced CYP1A1/1B1 expression was reduced in AhR RNAi clones. In the absence of 2,3,7,8-tetrachlorobenzo-p-dioxin, CYP1B1 mRNA levels were reduced in AhR RNAi clones, whereas CYP1A1 mRNA levels were barely detectable. Furthermore, anchorage-independent growth and intracellular oxidative stress were significantly reduced in AhR RNAi cells. Conclusions: In the absence of exogenous AhR ligands (such as cigarette smoke components), AhR overexpression up-regulated the expression of CYP1B1 in the early stage of lung adenocarcinoma. Elevated AhR expression in lung adenocarcinoma cells could increase intracellular oxidative stress and promote cell growth, implying that disrupting AhR expression might prevent the early development of lung adenocarcinomas.