An ionic (diatrizoate) and a non ionic (iopamidole) radiocontrast medium were compared as to their nephrotoxicity in a cohort of 21 patients with renal disease, 18 of which with normal renal function. The elevation of the urinary excretion of enzymes from renal tubular cells has been considered as a good index for renal tubular damage. We measured two lysosomal enzymes (NAG and beta glucuronidase) and a brush-border enzyme (gamma GT). In all patients we demonstrated an elevation of urinary excretion of the three enzymes already 12 hours after contrast medium administration. However, the elevation was statistically significant only after employment of the ionic contrast medium and concerned gamma GT and beta-glucuronidase; the urinary excretion of NAG did not vary significantly. Urinary enzymes levels returned to basal values 36 hours after intravenous pyelography. In conclusion, iopamidole showed a lower nephrotoxicity with respect to diatrizoate, as demonstrated by the lower levels of urinary enzymes excretion reached after its use.