To investigate the outcome of Graves' thyrotoxicosis after antithyroid drug management, data from 81 patients, treated in Chang Gung Memorial Hospital at Taipei and Linkou from October 1981 to March 1990, were analyzed. The gender ratio of female to male was 59:22. The mean age of onset was 33.1 +/- 10.5(15-60) year-old. All the patients were treated with antithyroid drug (Thionamide group) for a duration of 11 to 63 months (mean +/- SD = 28.1 +/- 9.8 months). Forty of 81 patients (49.4%) were remained remission after up to 2 years of follow-up. Those patients relapse usually occurred within 2 years after discontinuation of treatment (34/41), and only one exceptional case relapsed after 3 years. Three conditions affected the relapse rate. Patients with larger goiter (grade II-III) and shorter duration of treatment (< 23 months) had a higher relapse rate than those-with smaller goiter (grade O-I) [29/46 vs. 12/35; chi 2 = 6.576, p = 0.010; p = 0.015 in stepwise logistic regression (LR)] and longer duration of treatment (> or = 23 months) (15/20 vs. 26/61; chi 2 = 6.316, p = 0.012; p = 0.020 in LR). Patients with higher pre-treated serum triiodothyronine (T3) level (T3 > or = 300 ng/dl) had a higher relapse rate than those with lower T3 level (T3 < 300 ng/dl) in univariate analysis (30/50 vs. 11/31, chi 2 = 4.601, p = 0.032), but no significant difference by LR (P = 0.094). Other clinical parameters including age, sex, past history, family history, thyroxine (T4) level, T3/T4 ratio, thyroid autoantibodies, staging of ophthalmopathy, responsiveness to thyrotropin-releasing hormone stimulation test at the end of treatment, and whether combined treatment with thyroxine had no significant difference between the relapse and remission groups. These data suggest: (a) patients with larger goiter (grade II-III had higher relapse rate; (b) most of the recurrent thyrotoxicosis patients relapsed within two years after drug withdrawal; (c) continuing treatment for more than twenty-three months produces better outcome; (d) patients with Graves' thyrotoxicosis should be followed up for at least three years after withdrawal of antithyroid drug.