To investigate the prognostic value of Her2/neu expression in differentiated thyroid carcinomas 103 patients were retrospectively investigated. All of them received surgical and an identical follow-up treatment. The patients with papillary and follicular thyroid cancer were further separated into two groups concerning their clinical development, including one group without distant metastasis (follow-up of minimum 8 years). The second group presented with distant metastases as a sign of an aggressive behaviour. Her2/neu was immunohistochemically detected on sections from formalin-fixed, paraffin-embedded tissues using c-erbB-2/Her-2/neu oncoprotein Ab-17 monoclonal antibody (mAb). In statistical analysis using the Mann-Whitney U-test and chi(2) test, Her2/neu protein overexpression was significantly correlated with prognosis. Both tumour entities without distant metastases showed significantly less cytoplasmic immunostaining than patients with development of metastases. Concerning the clinical outcome, Her2/neu overexpression may be regarded as a prognostic factor in differentiated thyroid carcinomas. Moreover, in addition to standard radio-iodine elimination therapy, application of Herceptin could lead to new successful therapeutic concepts for a number of patients with progressive thyroid cancer.