There is substantial evidence to show that correction of the causes of secondary forms of hypertension usually but certainly not always restores blood pressure to normal. We have analyzed the effect of age on the response of the blood pressure to the removal of the cause of hypertension in groups of patients with primary aldosteronism, renovascular hypertension, Cushing's syndrome and hypothyroid hypertension. In the patients studied, surgical or medical elimination of the mechanism responsible for the hypertension when the patients were below the age of 40 reduced diastolic blood pressure 1 to 2 years later to below 90 mm Hg in 24 of 25 patients. In the patients who were treated when they were over 40 years of age, blood pressure remained above 90 mm Hg after corrective treatment in 23 of 61 individuals (38%). In the 4 groups studied there was a significant correlation between age and post-treatment systolic and/or diastolic blood pressure. The findings indicate the importance of correcting secondary hypertension at an early age.