The case records of 262 patients who received 131I therapy during a 6-year period were analysed retrospectively. These included seventy-eight patients who received more than one therapeutic dose. The major indication for 131I therapy was thyrotoxicosis occurring in patients aged 40 years and over. The racial incidence in this series is striking. The effects on thyroid function of two low-dose schedules (4 mCi and 6 mCi) were compared. The major complication encountered was hypothyroidism. 4 mCi appears to be as effective as 6 mCi with the added advantage of producing less hypothyroidism.