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 or 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.