Objective: The expression of Ki67, a proliferation marker, and p27, a cyclin dependent kinases(CDKs) inhibitor, has been studied in various human neoplasms. This study was carried out to determine whether these markers are useful in distinguishing benign from malignant lesions of the thyroid or predicting biologic behavior of malignant lesions. Material and Methods: Using immunohistochemical techniques with monoclonal antibodies to Ki67 and p27, we analyzed the expression of Ki67 and p27 in various thyroid disorders(25 follicular adenomas, 47 follicular carcinomas, 16 papillary carcinomas, 20 adenomatous goiters and 40 normal thyroid tissues). The labeling indices(LIs) were determined by counting cells expressing these markers in 1000 cells per immunostained slide. Results: Neoplastic thyroid diseases showed higher expression of Ki67 and lower expression of p27 than non-neoplastic diseases(p<0.05). The expression of p27 was significantly different between follicular adenomas($LI=55.4{pm}5.7$) and follicular carcinomas($LI=23.2{pm}10.2$). There was, however, no significant correlation between the degree of Ki67 and p27labeling indices and types of carcinoma or clinical aggressiveness of diseases. Conclusion: The degree of Ki67 and p27 expression was useful in distinguishing between benign from malignant thyroid lesions, particulary between follicular adenoma and follicular carcinoma, but was not directly proportional to the tumor aggressiveness.