OBJECTIVE c-erbB-2 expression has been found to be a potential marker of aggressive biological behaviour in some tumours, but the role played by this oncoprotein in the development and maintenance of thyroid tumours is still controversial. Therefore our objective was to determine whether c-erbB-2 was overexpressed in a large retrospective series of human thyroid tumours, including both from follicular and C-cell differentiation. RESULTS We have studied 67 thyroid tumours (10 follicular adenomas, 11 follicular carcinomas, three anaplastic carcinomas, 25 papillary carcinomas and 18 medullary carcinomas and 16 metastases) by immunohistochemistry using an antigen retrieval method for paraffin-embedded material and a specific polyclonal antibody against the intracytoplasmic part of c-erbB-2 oncoprotein. There are marked differences in the pattern of c-erbB-2 immunoreactivity depending on the type of thyroid tumour. Thus, no expression of the oncoprotein has been found in follicular adenomas, follicular carcinomas and anaplastic carcinomas, but 52% of papillary carcinomas (membranous and diffuse cytoplasmic patterns) and all medullary carcinomas (granular cytoplasmic pattern) are immunopositive. CONCLUSIONS Our results indicate that overexpression of c-erbB-2 oncoprotein is easily identifiable by immunohistochemistry in paraffin sections of certain thyroid tumours after applying an antigen retrieval method. This study suggests that c-erbB-2 oncoprotein may play some role in disease progression in papillary and medullary thyroid carcinomas, but the significance of the different immunohistochemical patterns merits further investigations.