Twenty-five patients with newly diagnosed thyrotoxicosis participated in a double-blind study of the added effect of propranolol during tiamazol treatment. During the trial, the effect of treatment was ascertained by estimation at regular intervals of the clinical score (Crooks-Wayne index), free thyroxine index, serum triiodothyronine, serum reverse triiodothyronine, serum thyroglobulin and serum thyroglobulin antibodies. Both groups became euthyroid within two weeks. No statistically significant difference in the values for the clinical score or for any of the laboratory parameters measured was found between propranolol- and the placebo-treated patients. It is concluded that propranolol has no beneficial effect on tiamazol-treated hyperthyroid patients.