A prospective study was undertaken to determine the incidence of spontaneous extravasation of contrast in 82 patients with acute renal colic. The extravasation rate among 37 patients receiving 75 ml of contrast was 8.1%, while the rate among 45 patients receiving either 300 ml of 30% solution by infusion or a 140 ml bolus dose was 24.4%; overall the rate was 17%. Thus the incidence of extravasation was shown to increase with higher doses of contrast media. The primary dose in acute renal colic or other forms of obstructive uropathy should be in the low range (20-25 g 1-); reinjection after screening can be undertaken if necessary.