Sentinel lymph node excision in breast cancer is a minimally invasive diagnostic procedure for accurate staging of the axilla and for avoiding unnecessary axillary dissection. In patients with palpable breast cancer we injected microcolloidal particles of human serum albumin labelled with technetium-99m the day before surgery. The sentinel node was detected intraoperatively with a handheld gammaprobe and then removed. Complete axillary dissection was performed and the nodes inspected by routine histological examination. The axillary lymph node status was correctly predicted by the sentinel node technique in 32 of 33 breast cancer patients. Two cases of micrometastases escaped routine histopathological detection but were identified by immunohistochemical analysis applying the antibody AE1/AE3 to pancytokeratins. Immunohistochemical examination of the sentinel node improves the diagnostic security of patients with breast carcinoma by detection of micrometastases.