The malignant phase of hypertension is invariably fatal unless treated, and rapid reduction of arterial pressure is thought to be the treatment of choice. Ten patients with accelerated hypertension are described in whom abnormal neurological signs developed following the rapid reduction of arterial pressure. Three patients died without recovering from the neurological damage. A fourth died of an unrelated cause a month later. Areas of ischaemic damage were found in the brains of three of these cases. Of the six survivors, four were left with some permanent neurological disability. It is likely that these changes resulted from the inability of the cerebral circulation in patients with severe hypertension to autoregulate blood flow to the brain, so that a rapid reduction in arterial pressure led to ischaemia, especially of the watershed areas of the brain. Cerebrovascular autoregulation is likely to be compromized in patients with cerebral oedema, stenosis of major cranial vessels or in those patients with long-standing severe hypertension. It is suggested that the blood pressure in patients with accelerated hypertension should be lowered gently over a period of several hours or even days in order to allow time for the cerebrovascular autoregulatory mechanisms to recover.