The correlation between Endocrine Exophthalmos and serum levels of EPS was investigated in 64 exophthalmic patients only 2 patients had no demonstrable EPS in their serum. Thirty-one of these patients were suffering from thyrotoxicosis, 11 from Cushing's syndrome and 2 from acrome-galy. Twenty were euthyroid. The serum of 44 non-exophthalmic subjects was tested 11 with thyrotoxicosis, 6 with Cushing's syndrome, 4 acromegalics and 23 normal subjects. Only in 2 of the non-exophthalmic thyrotoxics EPS was found in the serum. Both developed severe ophthalmopathy later in the course of the disease. In all groups of patients a fair correlation was found between the severity of the exophthalmos and EPS levels in the serum. In 8 patients, EPS levels were determined serially during the course of the disease, in some before as well as after pituitary surgery or irradiatioa Changes in the EPS levels, corresponded well with the course of the ophthalmopathy. The correlation was better than that reported by some other investigators. In the authors opinion, these divergencies can be explained by differences in the method of EPS determination used. It is concluded that it is very probable that endocrine exophthalmos in Graves' disease as well as in Cushing's syndrome and in acromegaly, is caused by an anterior pituitary factor (EPS).