Patients with both exophthalmos and hyperthyroidism were treated with different modes of therapy for their hyperthyroidism. Propylthiouracil followed by surgery, propylthiouracil followed by radioactive iodine, propylthiouracil alone, and radioactive iodine alone were used. Some of the patients became hypothyroid and were made euthyroid with levothyroxine sodium. Based on the mode of therapy and whether or not hypothyroidism occurred, each patient was assigned to one of seven groups and followed up for 18 months or longer. Careful exophthalmometry was performed at six-week intervals in all patients. Though progression of exophthalmos was noted in all groups, the group that received propylthiouracil demonstrated the greatest progression of exophthalmos. In the group receiving sodium iodide l 131 therapy and in the group treated surgically, the rate of progression of exophthalmos was lessened with the development of hypothyroidism. Since these hypothyroid patients were made euthyroid with supplemental levothyroxine, it appeared that loss of thyroid tissue, rather than the hypothyroidism per se, was responsible for the decrease in progression of the exophthalmos. The continued progression of exophthalmos in the propylthiouracil-treated group may be related to effects of propylthiouracil on the immune system.