Both anaplastic thyroid carcinoma (ATC) and angiosarcoma of the thyroid (AST) are highly aggressive malignancies with very limited therapeutic options. Since selective inhibition of COX-2, for example, by celecoxib has been shown to suppress both tumour formation and progression, we investigated COX-2 protein expression in a series of ATC and AST (26 cases each) using immunohistochemistry. COX-2 expression was demonstrated in 13 ATC (50%) and 11 AST (42%); a strong COX-2 expression in more than 50% of vital tumour cells was found in 5 ATC and 5 AST, respectively. Although a recently performed phase II trial applying celecoxib failed overall to halt tumour progression in differentiated thyroid carcinoma, the two cases with partial or complete remission noted in this study were related to tumours with immunohistochemically proven strong COX-2 expression. The strong COX-2 expression observed in approximately 20% of our ATC and AST samples may thus indicate selective patients with a possible therapeutic option for an otherwise fatal disease.