Is this thyroid hyperplasia true carcinoma? In view of the facts that young fish are more affected than old fish, that iodin stops the hyperplasia and causes it to return to the colloid state, that removal of the fish from the ponds to the open brook also affects a cure, that the clinical incidence of tumors is directly related to the water supply, and that the severity of the disease as determined by histological examination is likewise directly related to the water supply, we cannot accept the prevailing opinion that the disease is true cancer. On the other hand, we believe that the disease as studied by us is an extreme illustration of endemic goitre, the end stage of which is cretinism. The opinion that it is cancer is based purely on anatomical studies—the essential fact being that the growing thyroid invades bone, muscle, etc. Such findings in mammalian tissues are strong evidence of malignancy, but this standard cannot be applied to fish thyroids for the reason that any growth of a widely and loosely distributed nonencapsulated gland would give the histological appearance of invasion of the surrounding tissues. The general views as to the cause of the disease may be divided into two groups: (I) infectious and (2) metabolic. Taking up the infectious theory, there are abundant grounds for such a view when one considers the flora of the tank. The very careful histological examinations by Hofer, Bonnet and Pick have yielded uniformly negative results. Direct infection cannot be excluded, but the fact that fish placed in the tail-race recover spontaneously, although living in the theoretically most polluted water, is not in harmony with our present conception of water-borne infection. Taking up the metabolic theory, the three factors of overfeeding, overcrowding and limited proportional water supply together with the many possibilities into which they are divisible seem to us to be in some way, still obscure, directly concerned with the development of the disease. It is not probable that a single substance, whether the result of food decomposition or of excretory products, excites the thyroid to its activity but rather that the thyroid reaction is the result of the activity of a great variety of these products.