In recent years increasing prominence has been given to the fairly frequent occurrence of plasma cell tumors of extramedullary origin. The commonest situation for such tumors is in the nasal and nasopharyngeal mucosa and the associated cervical structures, but their occurrence in a number of other regions, such as the lip, jaw, scalp and anus, has been recorded. In the literature there appears to be no record of a plasmocytoma arising in the thyroid gland; consequently, the rarity of such a condition justifies a record of this case. The interest of the particular plasma cell tumor to be described is not in its mere rarity, however, but in the fact that it provides a clue linking the form of thyroid enlargement known as lymphadenoid goiter, or Riedel's chronic thyroiditis, with the extramedullary plasmocytomas. REPORT OF CASE A childless married woman aged 50 first noticed a swelling in the neck, in