This is a retrospective study designed to evaluate the initial response to carbimazole in patients with Graves' disease (GD), possible determinants of that response, the frequency of occurrence of adverse effects during treatment with carbimazole and the frequency of transient and permanent hypothyroidism after treatment with 131I in patients with GD and multinodular goiter (MNG). Data were collected from patients who first presented with GD or MNG at the Department of Endocrinology of the Royal Infirmary of Edinburgh between 1 January 1993 and 31 August 1996. Patients were divided into three groups: patients with GD treated with a daily dose of 40 mg carbimazole, patients with GD treated with a single dose of 400 MBq 1311, and patients with MNG treated with the same dose of 131I. Of the patients younger than 30 years, 50% remained biochemically hyperthyroid after 4-6 weeks of treatment with carbimazole, compared to 14% of patients over 30. Other determinants of the response to carbimazole expressed as the fall in thyroid hormone levels after 4-6 weeks were: pretreatment levels of FT4, T3, TRAb and the 4 h 131I uptake, patients with the higher levels responding significantly better to carbimazole. Adverse effects were reported in 11.5% of patients. Of the patients with GD treated with 1311, 62.6% became hypothyroid, transient hypothyroidism occurred in only 2.4% of these cases. The main predictors of development of hypothyroidism were positive titres of antithyroid peroxidase antibodies (AbTPO) and antithyroglobulin antibodies (AbTg), with positive predictive values of 79.5 and 91.6 respectively. None of the patients with MNG became hypothyroid after treatment with 131I, a response significantly different from patients with GD. In conclusion, GD younger patients might benefit from higher initial doses of carbimazole. In patients with positive titres of AbTPO and AbTg, lower doses of 1311 might prevent hypothyroidism. Transient hypothyroidism was underestimated in this study. No permanent thyroxin replacement therapy should be started within the first six months after 131I treatment.