Therapeutic effect of radio-iodine treatment on thyroid patients with autonomously functioning toxic thyroid nodule was evaluated. Fifty one patients were given a standard dose of 10 mci of radioiodine (I-131) and were followed up for 2-3 years. The failure rate (relapse after 10 mci of radioiodine) of this regime was 10%. It was found that the nodules less than or equal to 3 cms. in size were completely cured after a dose of 10 mic. of radio-iodine therapy, over a follow up period of next 6 months. Patients having nodules larger than 3 cms. relapsed after first dose of 10 mci of radio-iodine, but were cured completely after the second dose of 10 mci of radio-iodine therapy. Tri-iodothyronine (T3) and thyroxine (T4) values were both found to be high before giving treatment in all the cases. Only one case developed hypothyroidism after radioiodine therapy.