Germline mutations in p27 kip1 are associated with increased susceptibility to multiple endocrine neoplasias (MEN) both in rats and humans; however, the potential role of common polymorphisms of this gene in endocrine tumor susceptibility and tumorigenesis remains mostly unrecognized. To assess the risk associated with polymorphism rs2066827 (p27-V109G), we genotyped a large cohort of Brazilian patients with sporadic endocrine tumors (pituitary adenomas, n=252; pheochromocytomas, n=125; medullary thyroid carcinoma, n=51; and parathyroid adenomas, n=19) and 885 population-matched healthy controls and determined the odds ratios and 95% CIs. Significant associations were found for the group of patients with pituitary adenomas (P=0.01), particularly for those with ACTH-secreting pituitary adenomas (P=0.005). In contrast, no association was found with GH-secreting pituitary tumors alone or with the sporadic counterpart of MEN2-component neoplasias. Our in vitro analyses revealed increased colony formation and cell growth rate for an AtT20 corticotropin mouse cell line overexpressing the p27-V109G variant compared with cells transfected with the WT p27. However, the genotypic effects in genetic and in vitro approaches were divergent. In accordance with our genetic data showing specificity for ACTH-secreting pituitary tissues, the overexpression of p27-V109G in a GH3 somatotropin rat cell line resulted in no difference compared with the WT. Pituitary tumors are one of the major clinical components of syndromes associated with the p27 pathogenic mutations MENX and MEN4. Our genetic and in vitro data indicate that the common polymorphism rs2066827 may play a role in corticotropinoma susceptibility and tumorigenesis through a molecular mechanism not fully understood thus far.