Retrospective clinical studies of 211 thyreotoxic patients having received 131I-therapy were performed and processed by computer. The patients' mean age was 58 years, the male-female ratio 7.1 to 1. The incidence of symptoms and associated diseases was in agreement with data in the literature. Of the clinical symptoms, weight loss, weakness, fatigability, a fine tremor, decompensation and nervousness, called attention to the condition. Of the ECG changes, an absolute arrhythmia of atrial fibrillation and extrasystole may be indicative of hyperthyroidism. Clinically, there is an essential difference between juvenile and old-age thyrotoxicosis. Differences could also be noted between patients with toxic adenoma and those with non-toxic one. Toxic adenoma patients were more advanced in age and the female-male ratio was higher than in non-toxic cases. Absolute arrhythmia of atrial fibrillation, extrasystole, repolarization disorders, diabetes, hypertension and arteriocardiosclerosis occurred more often, while ophthalmopathy and immune disease were less frequent. The clinical picture may raise the suspicion of old-age thyrotoxicosis. Following laboratory diagnosis, treatment should be administered without delay.