Although an extensive literature exists concerning essential arterial hypertension, the primary etiology has been unclear. Arterial compression of the left lateral medulla oblongata by looping arteries of the base of the brain was seen incidently in 51 of 53 hypertensive patients who underwent left retromastoid craniectomy and microvascular decompression for unrelated cranial nerve dysfunctions. Such compression was not noted in normotensive patients. Treatment by vascular decompression of the medulla was performed in 42 of the 53 patients. Relief in the hypertension was seen in 32 of the patients and improvement in four. Arteriosclerosis and arterial ectasia contribute to arterial elongation and looping. If pulsatile compression of the left lateral medulla occurs, hypertension may develop as a consequence of an imbalance in the neural control systems that normally regulate blood pressure. The hypertension may further contribute to arterial elongation, providing a vicious circle of pathophysiologic changes.