Although headache is sometimes regarded as a symptom of severe hypertension, its relationship to blood pressure is not clear. In 11,710 patients with mild to moderate essential hypertension we have found that headache was common and may be reduced by treatment. We found a clear relationship between the frequency of reports of headache and both systolic and diastolic blood pressure irrespective of treatment or the type of drug used to treat hypertension. Calcium antagonists appeared most likely to be a cause of headache in hypertensive patients.