A total of 225 patients were treated for hyperthyroidism with 555 MBq (15 mCi) radioiodine to ablate the thyroid and induce early hypothyroidism. The efficacy of this treatment in eradicating hyperthyroidism and problems of follow up were assessed one to six years later from case records and questionnaires. Information was received from 197 out of 219 live patients (90%) and from 160 doctors concerning 207 patients (92%). Only three patients were not traced and six had died since treatment. The modal time to hypothyroidism was three months, and 64% of patients were hypothyroid at one year; 5.6% had failed to become euthyroid within one year. Ninety five per cent of patients had been seen by the doctor and 82% had had a thyroid test done within the past two years. Most doctors preferred patients to be returned to their care once thyroxine treatment was stabilised. An ablative dose of 131I is recommended as an effective means of treatment which has clear advantages over conventional methods. Good communications and effective follow up should ensure success.