Gefitinib (Iressa, ZD1839) is a potent high-affinity competitive tyrosine kinase inhibitor aimed primarily at epidermal growth factor receptor (EGFR). Inhibitors in this class have recently been approved for clinical use in the treatment of advanced non-small cell lung cancer as monotherapy following failure of chemotherapy. We examined the efficacy of gefitinib on lung tumorigenesis in mouse models using both postinitiation and progression protocols. Gefitinib was given at a dose of 200 mg/kg body weight (i.g.) beginning either 2 or 12 weeks following carcinogen initiation. In the postinitiation protocol, gefitinib significantly inhibited both tumor multiplicity (approximately 70%) and tumor load (approximately 90%) in A/J or p53-mutant mice (P < 0.0001). Interestingly, gefitinib was also highly effective against lung carcinogenesis in the progression protocol when individual animals already have multiple preinvasive lesions in the lung. Gefitinib exhibited approximately 60% inhibition of tumor multiplicity and approximately 80% inhibition of tumor load when compared with control mice (both P < 0.0001). These data show that gefitinib is a potent chemopreventive agent in both wild-type and p53-mutant mice and that a delayed administration was still highly effective. Analyses of mutations in the EGFR and K-ras genes in lung tumors from either control or treatment groups showed no mutations in EGFR and consistent mutation in K-ras. Using an oligonucleotide array on control and gefitinib-treated lesions showed that gefitinib treatment failed to alter the activity or the expression level of EGFR. In contrast, gefitinib treatment significantly altered the expression of a series of genes involved in cell cycle, cell proliferation, cell transformation, angiogenesis, DNA synthesis, cell migration, immune responses, and apoptosis. Thus, gefitinib showed highly promising chemopreventive and chemotherapeutic activity in this mouse model of lung carcinogenesis.