Therapy with anti-thyroid drugs is the initial option mostly used in our country for the treatment of hyperthyroidism due to Graves-Basedow disease. To evaluate the long term results of this kind of therapy, a total of 773 patients were studied who were diagnosed from 1975 to 1994 in three hospitals in Northern Spain (Hospital Central de Asturias, Hospital de Cruces and Hospital de Navarra) after a mean follow-up time after anti-thyroid drug withdrawal of 46 +/- 33.1 months. The results showed a likelihood of hyperthyroidism relapse of 42.9%, 59.8%, 67.9% and 78.9% at one, three, five and ten years, respectively. Goitre size was correlated very significantly with the likelihood of relapse (p < 0.0001). In contrast, only TBII positivity at the end of therapy among the remaining parameters (age, sex, goitre size, length of therapy, positivity of anti-thyroid antibodies and TBII) influenced significantly on the relapse likelihood (p < 0.05). In conclusion, after a long term follow-up after anti-thyroid therapy, a high relapse rate of hyperthyroidism in Graves-Basedow disease, which amounts up to 79% at ten years, was observed. Goitre size was the main predictive factor for this relapse.