Renal function was evaluated before and after intravenous urography (IVU) in 124 randomly selected patients. In cases with renal insufficiency (RI) (serum creatinine level, greater than or equal to 2 mg/dL), the incidence of renal impairment was higher (11 of 20 patients, 55%) than in the group without RI (16 of 104 patients, 15%). In the latter group, high blood pressure (BP) was associated with a higher frequency of renal impairment (28.6% vs 10.5%). Advanced age, mild proteinuria, and a single functioning kidney were not risk factors. The IVU preparation contributed to renal function impairment in ten cases, while in the other 17 cases, the iodinated contrast material was the only factor apparently involved. Renal function returned to its previous level in a mean period of 12 days. One patient suffered progressive and irreversible renal failure, and two others had a slight, persistent impairment of renal function. It was concluded that the incidence of renal function impairment is high, but cases are usually mild and reversible. The most important predisposing factors are RI and high BP.