AbstractAbsorbed doses to red marrow and blood from131I therapy have been re-evaluated using data which has become available more recently. Uncertainties in the calculations are discussed and a comparison is made with previously published dose estimates. The mean dose to red marrow from the treatment of hyperthyroidism is estimated to be 0.59 rad mCi−1, compared with 0.36 rad mCi−1for the euthyroid state.