OBJECTIVE To obtain a simple standard regimen, suitable for general practice, and based upon the addition of antithyroid drug plus thyroxine for attaining euthyroidism in patients with Graves' disease. METHODS Prospective, randomized trial of patients with Graves' disease followed for 3 months after the initiation of therapy with an antithyroid drug and combined with the later addition of triiodothyronine to keep the patient euthyroid. The patients were randomized, according to birth date, between methimazole and propylthiouracil. Three dose schemes were tested for each antithyroid drug. METHODS The study was performed at the thyroid outpatient units of two general hospitals, with the patients having been referred from primary care. METHODS Ninety-four patients with Graves' disease who were suitable for treatment with antithyroid drugs. METHODS The patients were allocated into six groups. Three groups received methimazole (10 mg every 6th, 8th or 12th h) and three received propylthiouracil (100 mg every 6th, 8th or 12th h). Twenty micrograms of triiodothyronine was added when the patients were euthyroid to avoid hypothyroidism. METHODS The lowest serum free thyroxine level within 3 months of the initiation of the antithyroid treatment. RESULTS Fourteen per cent of the patients on methimazole 10 mg every 12th h and 29% on propylthiouracil 100 mg every 12th h did not achieve euthyroidism within the 3-month observation period. All but one patient on methimazole 10 mg every 8th h or propylthiouracil 100 mg every 8th h reduced the free serum thyroxine levels to the normal or hypothyroid range within the observation period. All of the patients on methimazole 10 mg every 6th h and 56% on propylthiouracil 100 mg every 6th h reduced the serum T4 values into the hypothyroid range within the period. CONCLUSIONS A standard regimen, based upon the addition of methimazole 10 mg every 8th or 6th h or propylthiouracil 100 mg every 8th or 6th h and followed by the addition of thyroxine or triiodothyronine when euthyroid to avoid hypothyroidism, seems to be suitable for attaining euthyroidism within 3 months in patients with Graves' disease. A dose scheme based on methimazole 10 mg every 12th h or propylthiouracil 100 mg every 12th h were found to be unsuitable due to an unacceptably high incidence of failure to attain euthyroidism or hypothyroidism within 3 months.