HER-2/neu, also known as c-erbB-2/neu, is an oncogene located in chromosome 17 which encodes HER-2/neu, a transmembrane protein belonging to the EGFR family. The external domain of this protein is released by the cell and can be studied in serum by immunoassay. HER-2/neu in serum is a specific tumor marker and only slight elevations may be found in the absence of malignancy, mainly in association with liver diseases. Likewise, the highest concentrations of this oncoprotein are found in patients with breast cancer, but lower concentrations may be found in other malignancies, particularly ovarian, prostate and lung cancer (mainly adenocarcinomas). HER-2/neu assay sensitivity in patients with untreated primary loco-regional breast cancer is <10% and seems to be related to overexpression in tissue as well as to the most important prognostic factors: tumor size and nodal involvement. Serial HER-2/neu determinations after surgery seem to be useful in the early diagnosis of recurrence, mainly in patients with HER-2/neu overexpression in tissue, but additional studies are necessary to confirm these results. HER-2/neu sensitivity (proportion of patients with abnormal values) in patients with metastasis is around 40%-45%, with a clear relationship to tissue overexpression and to site (higher in visceral metastases) and number of metastases. The clinical utility of HER-2/neu in patients with advanced disease is mainly for therapeutic monitoring. Likewise, in most of the studies published, a relationship has been found between serum HER-2/neu levels (either pretreatment or at follow-up) with tumor response.