This study has been conducted to determine the diagnostic value of CA 15-3 in detecting metastatic breast carcinoma in pleural fluid. The material studied consisted of 115 malignant pleural effusions from invasive breast cancer patients. Expressions of CA 15-3, c-erbB-2, estrogen receptor (ER) and progesterone receptor (PR) were studied using immunocytochemistry. Of the cases studied, 115 were evaluated for CA 15-3, 70 for c-erbB-2, 100 for ER and 97 for PR. The 94% of the breast cancer cases studied showed a positive reaction with CA 15-3, while 31% for c-erbB-2, 20% for ER and 9% for PR were positive (p<0.01). The sensitivities of CA 15-3, c-erbB-2, ER and PR were 94%, 31%, 20% and 9%, respectively, for metastatic breast carcinoma. Our results indicate that CA 15-3 has the highest sensitivity for diagnosing malignant breast carcinomas in pleural fluids. [Turk J Cancer 2009;39(2):45-50]