A longitudinal study of sequential measurement of glomerular filtration rate and effective renal plasma flow was conducted over a 5-year period of antihypertensive drug treatment, in four adult patients admitted to hospital with previously untreated malignant essential hypertension. Treatment produced a substantial and sustained lowering of systemic arterial blood pressure, compared with grossly elevated pretreatment levels. Nevertheless, glomerular filtration rate and effective renal plasma flow remained relatively stable in all four patients, at roughly the same level at which they were admitted. It was concluded that effective therapy for hypertension may not necessarily reverse the vascular changes of arteriolar fibrinoid necrosis, which characterize malignant essential hypertension. Permanent damage to the renal arterioles, and the territory of kidney supplied by these vessels, may be a feature of this condition.