It has been established practice for 60 years to prepare thyrotoxic patients undergoing thyroidectomy with Lugol's iodine. However, evidence in support of its claimed benefits, namely a reduction in the vascularity and friability of the toxic thyroid gland, is scanty. We have therefore determined the effect of Lugol's iodine on thyroid blood flow, as measured by thyroid uptake of thallium-201, in nine patients with Graves' disease and one euthyroid patient. Thallium-201 uptake, as well as serum thyroxine and triiodothyronine, fell significantly after treatment with iodine. Although not correlated with thyroid function tests, thallium-201 uptake was significantly correlated with thyroid weight. These results support the contention that thyroid blood flow is reduced in thyrotoxic patients treated with Lugol's iodine.