Background : Differential diagnosis of thyroid nodule is important in deciding treatment modality and fine needle aspiration is a good method to do so. But, sometimes, it has limitation in use because of inadequate test material and difficulty in interpreting it. Among the study of oncogene and tumor suppresor gene on the origin of thyroid tumor, expression of Fra-1, one of AP-1 system, is increased in thyroid neoplasm. So there is a possibility that it would be used as a method for differential diagnosis of thyroid nodule. We tried to know whether presence or absence of Fra-1 expression can be used as a diagnostic method in differential diagnosis of thyroid nodule using immuno- histochemical(IHC) staining method.Methods : In 4 types of thyroid tumor that was confirmed by histologic diagnosis after operation(30 cases of papillary cancer, 10 cases of follicular cancer, 16 cases of follicular adenoma, 18 cases of adenomatous goiter), IHC staining method was performed to evaluate the expression of Fra-1.Results : In papillary and follicular thyroid cancer, the expression of Fra-1 was stronger than benign thyroid tumor, but there was no difference in Fra-1 expression between two types of carcinoma. Weak expression of Fra-1 was observed in all cases of follicular adenoma, and it was also weakly expressed in 6 out 18 cases of adenomatous goiter. Conclusion : The expression of Fra-1 was stronger in thyroid cancer than in benign thyroid adenoma, but it was impossible to differentiate thyroid cancer from benign thyroid adenoma by the presence or absence of Fra-1 expression using IHC staining method.(Korean J Med 59:398-403, 2000)