488 PATIENTS WITH Graves' disease were treated by 131Iodine between 1960 and 1974. 427 (87,5%) of these patients were reexamined several times (clinically, 131I-uptake, PB127I, T4 (CPB-A), T3-uptake, and since 1973 TRH-test). The 131I was given as an individually calculated single dose treatment, using 7 000 -- 10 000 rd before 1965 and 6 000 rd thereafter. Two thirds of the patients became euthyroid after a single 131I-dose. In 20% the treatment had to be repeated. These patients show evidently a different biological behaviour of their disease, since multiple treatments revealed a higher rate of failure (33--35%). There is no principal difference between the out-come after 131I-therapy and surgery concerning the rate of failure, respectively relapse (3--4%) and hypothyroidism. Early incidence of hypothyrodism is dose--dependent, as could be shown in patients treated with higher doses before 1965. The reduction of the irradiation dose to 6 000 rd was followed by a drop of hypothyroidism from 18% to 7%. The reasons of late incidence of hypothyroidism are discussed. The incidence of hypothroidism was calculated by three different methods (over-all incidence, incidence within the observed interval after therapy, life-table method). All three methods revealed different results. This has to be taken into account comparing results after radioiodine as well as after surgery. Radioiodine therapy for hyperthyroidism offers a true alternative to surgery.