Carcinoma antigen 19-9 (CA 19-9) and carcinoembryonic antigen (CEA) expression was immunohistochemically investigated in 48 cases of subacute granulomatous (de Quervain's) thyroiditis, two of focal lymphocytic thyroiditis, three of Hashimoto's thyroiditis, two of Graves' disease, and seven follicular adenomas, 27 follicular carcinomas, and eight papillary carcinomas of the thyroid. CA 19-9 expression was found in all cases of subacute thyroiditis, lymphocytic thyroiditis, and papillary carcinomas examined and in approximately 50% of follicular adenomas and carcinomas. The strongest CA 19-9 staining was demonstrated in late stage subacute thyroiditis and in papillary carcinomas with marked sclerosis. Occasionally CA 19-9 expression was present in seemingly normal thyroid parenchyma adjacent to the thyroid lesions investigated. CEA was found in the center of the granulomatous lesions in acute stage subacute thyroiditis. All neoplasms were CEA negative. CA 19-9 and CEA could be demonstrated occasionally in multinucleated giant cells of subacute thyroiditis, which may suggest that these giant cells are of either histiocytic or follicular cell origin. Immunohistochemical investigation with antibodies against CA 19-9 and CEA may help to histomorphologically define subacute granulomatous thyroiditis.