The Her2/neu expression in poorly-differentiated thyroid carcinomas (PDTC) and in anaplastic thyroid cancer (ATC) was investigated retrospectively. Tumors of 25 patients suffering from PDTC and 25 from ATC were evaluated, including 15 cases of PDTC with highly-differentiated tumor parts. Her2/neu levels were immunohistochemically detected on formalin-fixed, paraffin-embedded tissues using c-erbB-2/Her2/neu Oncoprotein Ab-17 monoclonal antibody(mAb). In statistical analysis using 1-way ANOVA, Her2/neu protein overexpression was highly significantly correlated with the differentiation status of the investigated tumor parts. Whereas ATCs showed only few positive tumor cells, PDTCs presented with large reacting tumor areas and highly-differentiated parts of PDTCs revealed the highest staining intensity. These findings suggest a decreasing impact of Her2/neu oncoproteins correlating to dedifferentiation, reflecting the aggressive biological behaviour of these tumors.