Eleven patients with malignant arterial hypertension and malignant nephrosclerosis, verified by percutaneous renal biopsy, were examined during an observation period of 47 months (median). All patients received antihypertensive therapy. Patients were divided into three groups with regard to development of renal function during the observation period. In two patients (group I), the integrated average blood pressure (IA-BP) was 134/95 mmHg (median) and the calculated creatinine clearance (Ccr) increased from 19 to 56 ml/min (medians). In four patients (group II), IA-BP was 154/102 mmHg and Ccr remained stable-at approximately 60 ml/min. In five patients, (group III), IA-BP was 189/114 mmHg, owing to lack of patient compliance and absence from regular control, and Ccr was reduced from 26 ml/min to less than 5 ml/min. During the first 2 months of antihypertensive therapy, renal function was temporarily reduced in 7 of the 11 patients. It is concluded that effective blood pressure control in patients with malignant arterial hypertension and malignant nephrosclerosis can be accompanied by a considerable improvement, or at least a preservation of renal function.