Prospective studies of Graves-Basedow disease in Spain are scarce. Our objective was to evaluate the clinical and biochemical evolution of a cohort of patients with Graves-Basedow disease.202 patients with Graves-Basedow disease diagnosed between January 1997 and June 1999.5.9% of patients received 131I and 2.5% underwent surgery after treatment with methimazole. A relapse was observed in 23.3% patients. In the survival analysis, significant differences with regard to the rate of relapse were observed according to the goitre degree.In our study, all patients reached remission with methimazole. There was a high rate of relapse following discontinuation of therapy within the first months. Goitre size at the time of diagnosis was a significant determining factor of relapse.