Seventy eight patients with thyrotoxicosis were reexamined from 1 to 7 years after treatment with radioactive iodine. Determinations of protein-bound iodine in the serum (SPBI) were carried out in all patients. Twenty-nine of the 78 were patients with recurrent thyrotoxicosis following previous surgery. In this group, previously operated upon, the incidence of hypothyroidism, both clinically and by SPBI determination, was somewhat higher than in the patients without previous surgery. Dissociation between clinical state and SPBI was noted in 34 of the 78 patients. Of 58 patients clinically judged to be euthyroid, 21 had low SPBI values. The incidence of low SPBI values progressively increased up to the third year of follow-up. Approximately 50% of the patients followed from 3 to 7 years after therapy had low SPBI values.