A group of 375 untreated euthyroid patients with solitary autonomous adenoma of the thyroid were studied in a long-term follow-up (observation period 52.8 (mean)/46 (median), range 3–204 months). During the period of observation, 133 (34.2%) of all initially untreated patients underwent treatment (surgery, radioiodine, antithyroid medication) because of hyperthyroidism, mechanical problems, or at the patient's request. Sixty-seven patients developed hyperthyroidism resulting in a mean incidence of 4.1% per year. The incidence of hyperthyroidism increased during follow-up (3% in the first seven years, 10% in the following years). Age, sex, nodule size, initial scintigraphic appearance and the TRH test were of no individual prognostic value in predicting hyperthyroidism. Eleven of 14 patients with untreated hyperthyroidism became euthyroid without treatment during the time of follow-up. After iodine excess (by history or elevated iodine levels in urine, N=45), 14 patients (31%) developed hyperthyroidism. In conclusion, we recommend a definitive treatment of autonomous adenoma at least in patients with advanced age, concomitant diseases and a higher probability of iodine exposure.