An analysis was made of observations carried out in 65 patients with ophthalmopathy of Graves' disease, in an attempt to identify any statistical correlation between the level of long-acting thyroid stimulator in the blood of the patients, the severity of opthalmopathy and the incidence of preceding radioiodine therapy. Thirty-five of the sera did not give a positive response in the bioassay for the thyroid stimulator; with 17 a positive response up to 300% at 9 hr with 0.5 ml/mouse was found; and with the remaining 13 the value was greater than 300%. There was no correlation between these values and the severity of ophthalmopathy or the assessment of whether or not the eye condition was progressing at the time that serum was obtained. There was no correlation between the incidence of preceding radioiodine therapy and the severity of ophthalmopathy or the value obtained on assay of serum for the long-acting thyroid stimulator. With one additional patient, observations over a period of 15 months showed that exophthalmos progressed while the level of the long-acting thyroid stimulator in the patient's blood fell; coincident with the decrease in concentration of the long-acting thyroid stimulator the patient's hyperthyroidism went into remission and subsequently hypothyroidism developed. The conclusion was that the correlation between the presence or degree of ophthalmopathy and the incidence of the longacting thyroid stimulator was insufficient to show that the latter had a major role in maintaining the ophthalmic syndrome, and it is difficult to see it as a direct cause of ophthalmopathy.