The effectiveness of methimazole used in a single daily dose was studied in 47 patients during their initial episode of hyperthyroidism, and in an additional eight patients during a relapse of hyperthyroidism. All patients become euthyroid using this method. Mean time required to achieve a euthyroid state was 16.7±1.1 weeks in the former group, and 14.9±2.9 weeks in the latter. In one patient there was an initial response to single daily dose therapy, but subsequently split dosage was required for control. The single daily dose regimen of antithyroid drugs will control hyperthyroidism in most patients. We have found that propylthiouracil will achieve this more rapidly than methimazole.