Purpose: Cyclin-dependent kinase inhibitor, p27kip1 is an important cell-cycle regulator. In recent studies, the loss of p27kip1 expression has been examined in many neoplasms including colorectal cancer and breast cancer. The aims of this study are to evaluate the correlation between p27 expression and carcinogenesis of the thyroid giand and to determine its clinical applications. Methods : Immunohistochemical staining for p27kip1 was performed in 10 nodular goiters, 12 follicular adenomas, 8 minimally invasive follicular carcinomas, 6 widely invasive follicular carcinomas, and 26 papillary carcinomas. Results : p27kip1 was expressed in 90.0% (9/10) of nodular goiters, 66.7% (8/12) of follicular adenomas, 62.5% (5/8) of minimally invasive follicular carcinomas, 33.3%(2/6) of widely invasive follicular carcinomas, and 46.2% (12/26) of papillary carcinomas (P>0.05). The expression of p27kip1 was not found to be correlated with tumor size in either neoplasms or nodal metastasis in papillary carcinoma. Conclusion : These results suggest that p27kip1 does play some role in the tumorigenesis of thyroid cancer but is no a prognostic factor. (J Korean Surg Soc 2001;60:386-390)