Mutant mice lacking both cyclin-dependent kinase (CDK) inhibitors p18(Ink4c) and p27(Kip1) develop a tumor spectrum reminiscent of human multiple endocrine neoplasia (MEN) syndromes. To determine how p18 and p27 genetically interact with Men1, the tumor suppressor gene mutated in familial MEN1, we characterized p18-Men1 and p27-Men1 double mutant mice and showed that p18, but not p27, functionally collaborates with Men1 in suppressing lung tumorigenesis. Lung tumors developed in both Men1(+/-) and p18(-/-);Men1(+/-) mice at a high penetrance and contain both neuroendocrine and nonneuroendocrine cells. The remaining wild-type Men1 allele was lost in most lung tumors from Men1(+/-) mice but was retained in most tumors from p18(-/-);Men1(+/-) mice, showing a functional collaboration between p18 and Men1 in lung tumor suppression. Phosphorylation of Rb protein at both CDK2 and CDK4/CDK6 sites were significantly increased in normal bronchial epithelia and tumor cells derived from p18(-/-);Men1(+/-) mice compared to those from single p18(-/-) or Men1(+/-) mice. Lung tumors developed in p18(-/-);Men1(+/-) mice were multifocal, more heterogeneous, and highly invasive compared to those developed in either p18(-/-) or Men1(+/-) mice. Bronchioalveolar stem cells are expanded in normal and tumorigenic lungs of p18(-/-) mice and are further expanded in p18(-/-);Men1(+/-) lung tumors. These results reveal a previously unrecognized function of p18 in lung tumor suppression through collaboration with Men1 to control lung stem cell proliferation.