Previously, we demonstrated that 8-methoxypsoralen (methoxsalen), a potent human cytochrome P450 2A6 (CYP2A6) inhibitor, strongly suppresses lung adenoma induction by 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) in female A/J mice. In the present study, we examined the inhibitory effects of methoxsalen on the development of lung adenocarcinomas, as well as on adenomas and alveolar hyperplasia. Female A/J mice were treated with methoxsalen at doses of 12.5 or 1.25 mg/kg body weight, administered by stomach tube once daily for 3 days. One hour after the final treatment, NNK was injected i.p. at a dose of 2 mg/mouse. The experiments were terminated 52 weeks after the first methoxsalen treatment, and lung adenomas and adenocarcinomas were analyzed histopathologically. Pretreatment with methoxsalen significantly reduced the incidence of adenocarcinomas from 94.7 to 46.7% (12.5 mg/kg) and 44.4% (1.25 mg/kg), and their tumor multiplicity from 4.68 to 0.87 (12.5 mg/kg) and 0.61 (1.25 mg/kg) tumors/mouse. The tumor multiplicity of adenomas and adenocarcinomas in the methoxsalen-treated groups was significantly reduced from 12.47 to 5.67 (12.5 mg/kg) and 4.28 (1.25 mg/kg) tumors/mouse. Approximately 60% of the adenocarcinomas arose within adenomas. In comparing the methoxsalen + NNK and NNK alone groups, there was no significant difference in the frequency of such compound lesions, indicating that pretreatment with methoxsalen did not suppress the eventual progression of adenomas to adenocarcinomas. These results clearly demonstrate that methoxsalen, a potent human CYP2A6 inhibitor, inhibits not only lung adenoma but also adenocarcinoma development.