In a prospective study with a median follow-up of 13 months on a series of 71 breast cancer patients, 9 developed haematogenous metastases. The neu protein was found on the cell membranes in 27 of the 71 carcinomas (38%) by an immunohistochemical technique using a monoclonal antibody (MAb). Eight of the 9 patients with haematogenous metastases showed overexpression of the neu protein. Immunohistochemical staining of the cell membrane was inversely correlated with the oestrogen and the progesterone receptor status. There was no correlation with lymph-node involvement. The immunohistochemical detection of the neu protein in breast adenocarcinomas is an independent factor in predicting the patients at risk for haematogenous tumour spread and is therefore correlated with unfavourable prognosis.