A group of 145 high-risk patients who underwent angiography after administration of the nonionic contrast agent iohexol were monitored for the development of acute renal dysfunction. The results in this group were compared with those in 202 high-risk historical control subjects who had undergone angiography after administration of ionic contrast material. All patients in both groups received similar pre- and postangiographic treatment. A greater number of patients in the ionic group had preexisting renal disease, were of advanced age, and had received large volumes of contrast material. Acute renal dysfunction occurred in 20 of the 202 (10%) patients in the ionic group, compared with eight of the 145 (5.5%) patients in the nonionic group; this difference is not statistically significant. Five patients in the ionic group, but none of the patients in the nonionic group, ultimately required dialysis; this difference is not statistically significant. The findings suggest that a randomized trial in high-risk patients should be undertaken before a clinical advantage of the nonionic contrast agent iohexol with regard to renal function can be assumed.