We evaluated deguelin and silibinin in A/J mice treated with the tobacco-specific carcinogen benzo(a)pyrene (BP) for their ability to inhibit pulmonary adenoma formation and growth. Animals were treated with either deguelin (5.0 or 10.0 mg/kg body weight, by gavage) or silibinin at doses of 0.05% and 0.1% in the diet, approximately 10 days before a single intraperitoneal dose of BP. We found that oral administration of deguelin reduced tumor multiplicity by 56% and tumor load by 78%, whereas silibinin treatment at doses of 0.05% and 0.1% in the diet did not show any significant efficacy on either tumor multiplicity or tumor load. The result indicates that deguelin significantly inhibits pulmonary adenoma formation and growth in A/J mice. Finding new and effective agents that can prevent lung cancer is urgently needed because cancer of the lungs remains the principal cause of cancer deaths in the United States and because effective chemoprevention of this cancer type remains elusive. Thus, deguelin appears to be a promising new preventive agent for lung cancer and may be considered for further studies in other animal models and in clinical trials.