The renal status of 38 patients with progressive systemic sclerosis (scleroderma) has been investigated by the usual clinical tests, urine electrophoresis, glomerular filtration rate (GFR) and renal plasma flow (RPF) determinations and in 4 cases by renal biopsy. Fourteen patients presented with proteinuria and/or a high serum creatinine and/or hypertension with low clearance values in all cases. In 14 other patients, an abnormality was apparent from clearance results (12 cases), renal biopsy (1), urine electrophoresis (1). The earliest sign of renal involvement that could be demonstrated was a reduced RPF and an elevated filtration fraction. Subsequently, a glomerular proteinuria with a electrophoretic pattern was observed as either the only sign (9 cases) or in association with abnormal clearance values (8 cases). The incidence of clinical renal involvement (proteinuria, renal failure, hypertension) ranged from 16 to 60%; 2/3 of patients who presented with proteinuria and hypertension died within 3 years. A mucoid thickening of intima and a fibrosis of adventitia in the proximal part of interlobar and arciform arteries, a fibrinoid necrosis in the distal part of lobular and preglomerular arteries are distinctive although inconstant features. The vascular lesions (seen in 70% of cases) and superimposed but reversible vasoconstriction, account for the decreased RPF. An effective control of blood pressure is mandatory; the therapeutic value of angiotensin converting enzyme inhibition remains to be corroborated.