Using computed tomography, the renal parenchymal volume and tissue-plasma ratio of contrast enhancement, with a bolus injection, were measured, and the latter measurement is believed to indicate regional renal function per unit of parenchymal volume. 67 studies were carried out on 50 patients, 17 of whom were examined pre- and postoperatively. Of these 50 patients, 27 had hydronephrosis, 6 had renovascular disease, 4 had pyelonephritis and the remaining 13 were control patients. The product of tissue-plasma ratio of contrast enhancement and the calculated renal parenchymal volume were found to be in good correlation with the individual renal function measured with 99mTc-dimercaptosuccinic acid and 131I-hippuran. Since this method is reproducible, the kidney being examined may be seen in more detail.