To better define the effect of age on blood pressure in patients with hyperthyroidism, the blood pressures of 321 patients with hyperthyroidism were compared with those of 324 euthyroid controls. Subjects were divided into four age groups by decade from 20 to 59 years. The systolic, but not diastolic, blood pressure in the patients with hyperthyroidism was significantly higher in all age groups. Among the euthyroid controls, the systolic blood pressure increased with age. However, no age‐related increase was apparent among the hyperthyroid patients because of the relatively high systolic blood pressure in young patients. Effective antithyroid treatment reduced the systolic blood pressure significantly in hyperthyroid patients. The reduction of systolic blood pressure was greater in the younger hyperthyroid patients than that in the older ones, so that an age‐related increase in systolic blood pressure was present after a euthyroid state had been achieved. These results provide evidence against the report that in hyperthyroidism, hypertension tends to occur in older patients, and suggest that the effect of excessive thyroid hormone on the systolic blood pressure may differ according to age.J Am Geriatr Soc 33:19,