Malignant hypertension developed in an 18-year-old man whose primary hypertension had been diagnosed by chance. Standing blood pressure was 290/170 mmHg. Tests of renal function revealed high blood urea nitrogen and creatinine levels and low levels of both effective renal plasma flow and the glomerular filtration rate. Plasma renin activity and levels of angiotensin II and aldosterone were greatly elevated. Severe concentric left ventricular hypertrophy was noted. The patient received standard antihypertensive treatment with furosemide, propranolol, nifedipine, and prazosin, but his blood pressure did not decrease and there was no improvement in the clinical or biochemical measures. The patient was then given 20 mg of enalapril daily for one year. The inhibition of angiotensin converting enzyme immediately reduced blood pressure. Angiotensin II and aldosterone levels became normal, kidney function and hemodynamics improved, and echocardiograms revealed that the left ventricular hypertrophy had regressed. The results confirm the pathogenetic role of angiotensin II in the development of the malignant phase of hypertension.