Monoclonal antibodies against cytokeratin (AEI/AE3) were applied as a probe, using APAAP immunohistochemistry technique to detect 635 lymph nodes from 45 breast cancer patients with negative lymph nodes. Micrometastases were identified in 14 lymph nodes of 9 cases (20%). A significant difference was found between cytokeratin staining positive group and cytokeratin staining negative group in disease-free and over-all Kaplan-Meier survival curves. The detection of micrometastases had more clinical value for T₁ and T₂ patients. One of 2 T₁ cytokeratin positive cases relapsed while only 1 of 19 T₁ negative cases relapsed within 5 years; three of 5 T₂ cytokeratin positive cases relapsed while 1 of 17 negative cases did. The presence of micrometastases had the same value in predicting local recurrence and distant metastases.