Purpose: The purpose of this study was to determine the utility of tumor marker CA 15-3 in the following: the diagnosis of breast cancer relapse after curative mastectomy, and the differentiation of the value of tumor marker by site of metastases. Materials and Methods:　Two hundred two patients (median age 48 years) with breast cancer included in the follow-up after curative mastectomy. The tumor marker CA 15-3 was determined by IRMA (CIS BIO　INTERNATIONAL, France). Test values > 30 U/ml were considered elevated (positive). Results: Among 202 patients, recurrent diseases were found in 16 patients. CA 15-3 was elevated in 5 of 16 patients with recurrences. There was no false-positive patient who had elevated CA 15-3. Sensitivity and specificity of CA 15-3 for detection of breast cancer recurrence were 31%, and 100%. CA 15-3 was elevated in all of the 4 patients with liver metastases. CA 15-3 was elevated in none of the patients who relapsed with metastasis to bone-only or contralateral breast-only. Conclusion: The tumor marker CA 15-3 in the detection of breast cancer relapse after curative mastectomy is specific, but not sensitive. However, it is useful to rule out liver metastases of breast cancer, which indicates bad prognosis. (Korean J Nucl Med 38(4):311-317, 2004)