HER-2/neu oncogene codes the transmembrane growth factor receptor, HER-2/neu oncoprotein. Its overexpression of up to 30 % was proved in breast cancer patient primary tumor tissue. Extracellular domain of this oncoprotein can be found in circulation. Materiál, methods: We evaluated dynamics of sérum HER-2/neu changes (ELISA Oncogene Science, USA) in our group of 28 patients during Herceptin therapy. Results: Non-responding patients evaluated at the time of first restaging (usually 3 months) are characterized by significantly higher concentrations of sérum HER-2/neu which are always above cut-off level. Sérum HER-2/neu concentrations in patients with best clinical response characterized as complete or partial remission usually normalize within three months of therapy, even though the pretreatment levels were high. Sérum CEA and CAI5-3 levels do not show significant differences between these two patients groups. Sérum HER-2/neu during the initial three months of Herceptin treatment is also the best predictor of therapy failure in long-term follow up. Conclusion: Sérum HER-2/neu determination during Herceptin therapy appears to be a suitable therapy efficacy validation parameter both at the start and during long-term therapy application

        Key words: sérum HER-2/neu, breast cancer, Herceptin, follow-up