To cast light on relations of cyclooxygenase-2 (COX-2) expression to lymphocytic thyroiditis and thyroid tumorigenesis, protein levels were immunohistochemically assessed and compared with inducible nitric oxide synthase (iNOS) in a total of 181 cases: follicular adenoma, 23; well-differentiated papillary carcinoma, 85; poorly differentiated papillary carcinoma, 25; anaplastic carcinoma, 7; and follicular carcinoma, 41.