PURPOSE: The presence of lymph node metastasis is a key factor for deciding upon radical axillary dissection during a breast cancer operation. We performed prospective research to assess the accuracy and effectiveness of ex vivo ultrasound for detecting sentinel node metastasis during the operation. METHODS: 183 fresh sentinel lymph nodes from 30 breast cancer patients had sonographic examination performed on them by a specialized radiologist immediately after surgical resection. The sonographic criteria for malignant lymph nodes were uneven cortical thickness, a node more than 3 mm in diameter, the absence of the hilum and a round hypoechoic node. After sonographic evaluation, the specimen were delivered to the department of pathology for frozen biopsy and permanent staining. RESULTS: Among 133 lymph nodes that were examined on frozen sectioning, 12 (9.0%) were revealed as metastatic nodes and 121 (90.9%) were revealed as benign. On US examination, 150 (81.9%) among the detected 183 nodes were read as benign and 33 (18.1%) were read as metastatic. Among 150 benign nodes, four (2.6%) were proven as metastasis on permanent pathology, and 20 (60.6%) among the 33 sonographic cancerous nodes were reported as metastatic nodes. The sensitivity, specificity, positive predictive value, negative predictive value, false positive rate, false negative rate and accuracy of ex vivo ultrasound were 83.3%, 91.8%, 60.6%, 97.3%, 8.2%, 16.7% and 90.7% respectively. CONCLUSION: Ex vivo ultrasound evaluation for detecting sentinel lymph node metastasis during the operation may be helpful to decide the extent of lymph node dissection.