bcl-2 protein which protects cells from apoptosis is found to be expressed in several neoplastic and non neoplastic tissues. bcl-2 is consistently expressed in normal thyroid tissue but the regulation of its expression in thyroid tumors derived from the follicular cells remains to be discovered. In this study, we have assessed bcl-2 expression in different samples of non neoplastic and neoplastic thyroid tissues. Sixteen papillary carcinomas (including four papillary carcinomas of follicular variant), eight follicular carcinomas and five undifferentiated carcinomas were selected for this study. The tissues were routinely fixed and paraffin embedded. Four cases of normal thyroid tissue, four cases of Hashimoto's thyroiditis and five cases of thyroid tissue with Graves' disease were also investigated. Immunohistochemistry was performed with an anti-bcl-2 monoclonal antibody on paraffin sections. bcl-2 was highly expressed in normal follicular cells but also in follicles of Hashimoto's thyroiditis and Graves' disease while its expression appeared to be downregulated in papillary (14 cases on 16), in follicular (five cases on eight) and undifferentiated (five cases on five) carcinomas. However, in five carcinoma cases (one papillary, one papillary follicular variant, three of follicular type), bcl-2 expression was similar or higher than in normal tissue. The highest expression of bcl-2 was observed in one case of follicular carcinoma with large amounts of atypical cells. bcl-2 protein seems to play a more important role in non-neoplastic thyroid cells rather than in carcinoma cells. The heterogeneity of its expression in thyroid neoplasms opens the question as to whether bcl-2 detection could serve as a prognostic factor in such tumors. This question could be answered on large series of cases with concomitant analysis of clinical data.