SUMMARYThe course of thyrotoxicosis treated with antithyroid drugs has been studied in detail in 105 thyrotoxic patients. Each patient was seen at intervals of 1–3 months over a period of more than 4 years and thus an average of thirty serial tests of thyroid function were made on each patient. Forty‐three patients remitted after one course of antithyroid drugs. In another forty‐three patients the disease either required operative treatment or was still active 4 years after the start of therapy. The remaining nineteen patients fell into an intermediate group.In the light of the variable course of the disease, we conclude that most patients with Graves' disease under 45 years are best treated initially with antithyroid drugs. Reassessment of thyroid function at 6‐monthly intervals during and for a period after antithyroid drug therapy is useful and permits selection of the most appropriate method of tr