The duration of action of methimazole (MMI) was studied in patients with hyperthyroidism due to Graves' disease. Perchlorate discharge tests performed 24 h after MMI administration revealed greater than 10% discharge in 77% of 53 patients who received a single dose of 15 mg MMI and in 74% of 23 patients who received 30 mg. The mean percent discharges were 41.5 +/- 26.4% (+/- SD) and 35.4 +/- 28.0, respectively. Based on these results, hyperthyroidism was treated with a single daily dose (SDD) of 15 mg in 43 patients and with 30 mg in 32 patients, and the results were compared with retrospective analysis of 50 patients who were treated with divided doses of MMI (10 mg, 3 times daily). Within 12 weeks, 93% of the patients treated with 15 mg SDD, 91% treated with 30 mg SDD, and 86% treated with divided doses were euthyroid. The mean times to achieve euthyroidism in these patients were 5.3 +/- 3.6 (+/- SD), 5.3 +/- 3.1, and 5.6 +/- 3.0 weeks, respectively. Side-effects occurred in 2 patients treated with 15 mg SDD and in 6 treated with 30 mg SDD. We conclude that a single daily dose of 15 mg MMI is not only effective in most patients with Graves' hyperthyroidism, but also less frequently causes adverse effects.