SINCE the original description of struma lymphomatosa by Hashimoto in 1912, much confusion has arisen in the literature concerning the diagnosis, pathology, and therapy of this clinicopathologic entity. The present study is a review of 51 cases of struma lymphomatosa seen at the Massachusetts General Hospital. Since the literature is lacking in reports of some of the newer laboratory methods of studying thyroid function in struma lymphomatosa, it is one of the purposes of this paper to describe such observations. Serum protein-bound-iodine determinations, radioactive-iodine (I131) tracer studies, and radioautographs were obtained on several of the patients in the present series. These help to throw additional light on a number of current problems. The etiological basis of struma lymphomatosa is unknown. Various theories have been advanced to explain the picture of this entity. These hypotheses will be mentioned later in the section under Comment. METHODS AND CLASSIFICATION OF MATERIAL There