It is disputed to what extent tumor necrosis factor-a is present in the thyroid follicular epithelial cells and/or in the interstitial cells in different disorders of the thyroid gland.We describe the immunohistochemical detection of tumor necrosis factor-αusing formaldehyde fixed and paraffin embedded tissue and a polyclonal anti-serum with high tumor necrosis factor-αneutralising activity. We examined the distribution of tumor necrosis factor-αin interstitial cells and follicular epithelial cells in thyroid carcinomas, adenomas, non-toxic multinodular goiters and autoimmune thyroid diseases.Tumor necrosis factor-αwas demonstrated in thyroid follicular epithelial cells, most frequently in non-toxic multinodular goiters (six of seven patients) and less frequently in adenomas (three of nine patients), papillary carcinomas (two of five patients), follicular carcinomas (one of five patients), Hashimoto's disease (one of six patients) and Graves' disease (one of seven patients). Tumor necrosis factor-αproducing interstitial cells were found in two thirds of patients with all six thyroid diseases.