An unusual case of a microfollicular adenoma of the thyroid gland with signet-ring cells in a 36-year-old euthyroid man is described. Mucin stains demonstrated the presence of mainly neutral glycoproteins with focal participation of acidic groups. Immunohistochemical staining for thyroglobulin was strongly positive. The apparent "mucin secretion" described in these rare neoplasms could be due to the histochemical staining of carbohydrate components or breakdown products of thyroglobulin and colloid. The published work on primary mucin secreting tumors of the thyroid gland is reviewed.