Elevated carbohydrate antigen (CA) 19-9 levels are frequently associated with occult neoplasm. High CA 19-9 values trigger a search for underlying malignancy. Although malignant thyroid disease has been associated with high CA 19-9 levels in several studies, an unusual case of thyroid follicular adenoma associated with marked increase in CA 19-9 level with confirmed immunocytochemical positivity for CA 19-9 is presented.