A 49-year-old female with a 30-year history of untreated essential hypertension was noted to have a blood pressure of 290/175 mmHg during evaluation for elective gynecological surgery. At the time of hospitalization she complained chiefly of chronic frontal headaches. Physical examination revealed grade two hypertensive retinopathy, and laboratory studies showed left ventricular hypertrophy. Over the next 12 days the patient's blood pressure was successfully lowered to 178/106 mmHg. During revision of her therapy her mean blood pressure rose to 244/144 mm Hg (88% of the admission level) over 36 hours and she developed hypertensive encephalopathy with papilledema, headaches and projectile vomiting. Concomitant resolution of neurological symptoms and control of blood pressure occurred over the next nine days. The course of this patient suggests that autoregulation of cerebral blood flow may be acutely reversed and that the occurrence of hypertensive encephalopathy depends not only on the magnitude and duration of the blood pressure elevation but, more important, on the rate at which that blood pressure is attained.