Background. This study was designed to ascertain whether immunohistochemical methods could improve the detection of metastases in primary breast-cancer patients whose axillary lymph nodes were classified, by conventional methods, as disease free. Methods. Ipsilateral lymph nodes (negative for metastases by routine histology) from 736 patients (participants in Trial V of the International [Ludwig] Breast Cancer Study) were examined by serial sectioning and staining with haematoxylin and eosin (two sections from each of six levels) and by immunohistochemistry of a single section (with two anticytokeratins AE-1 and CAM 5.2). After median follow-up of 12 years, disease-free and overall survival were estimated by Kaplan-Meier methods. Findings. Occult nodal metastases were detected by serial sectioning and haematoxylin and eosin in 52 (7%) of 736 patients and by immunohistochemistry in 148 (20%). Only two (3%) of 64 invasive lobular or mixed invasive lobular and ductal cancers had node micrometastases, detected by haematoxylin and eosin, compared with 25 (39%) by immunohistochemistry. Occult metastases, detected by either method, were associated with significantly poor disease-free and overall survival in postmenopausal but not in premenopausal patients. Immunohistochemically detected occult lymph-node metastases remained an independent and highly significant predictor of recurrence even after control for tumour grade, tumour size, oestrogen-receptor status, vascular invasion, and treatment assignment (hazard ratio 1.79 [95% CI 1.17-2.74], p = 0 007). Interpretation. The immunohistochemical examination of ipsilateral axillary lymph nodes is a reliable, prognostically valuable, and simple method for the detection of occult nodal metastases. Immunohistochemistry is recommended as a standard method of node examination in postmenopausal patients.