Objective To detect the expression of PCNA and Ki-67 proliferation antigens in thyroid neoplasms and to study diagnostic value of expression of PCNA and Ki-67 in benign and malignant neoplasms of thyroid. Methods The expression of PCNA and Ki-67 in 76 thyroid neoplasms was examined by immunohistochemical method(EnVision MT ). Results Among the 39 thyroid adenomas(TA), the indices of PCNA and Ki-67 were 0 16?0 09(39/39) and 1 49?1 15(39/39) respectively; Among the 37 thyroid carcinomas(TC), the indices of PCNA and Ki-67 were 0 43?0 31(37/37) and 2 13?1 52(37/37) respectively. The differences of both the PCNA and Ki-67 indices between TA and TC were significant(P