Patients with renal scarring due to previous upper urinary tract infections (UTI) are at risk of developing hypertension and renal insufficiency. In this study glomerular filtration rate (GFR), systolic (SBP) and diastolic (DBP) blood pressure, peripheral renin activity (PRA), plasma (p-Aldo) and urine aldosterone (u-Aldo), the urinary excretion of sodium (UNa) and potassium (UK) and the fractional sodium (CNa/CIn) and potassium (CK/CIn) excretion were determined in 22 female patients with verified renal scarring and a history of febrile UTI and in nine age-matched healthy women with normal i.v. urograms. The patients had significantly lower GFR, higher SBP, higher PRA and higher CK/CIn than the healthy controls. A significant correlation between CNa/CIn and SBP (r = 0.51, P less than 0.05), DBP (r = 0.50, P less than 0.05) and PRA (r = -0.47, P less than 0.05) was found. The ratio of UK/UNa was significantly correlated to SBP (r = -0.43, P less than 0.05), DBP (r = -0.44, P less than 0.05), PRA (r = 0.65, P less than 0.01) and p-Aldo (r = 0.68, P less than 0.01). It is concluded that the renin-aldosterone system probably is involved in the pathogenesis of hypertension in patients with pyelonephritic renal scarring and that screening determinations of UNa and UK may prove useful for detection of individuals with increased PRA and p-Aldo.