The safety and diagnostic efficacy of iopentol 300 mg I/ml were compared with iohexol 300 mg I/ml in 300 patients submitted for urography. The study was carried out as a double-blind, randomised parallel study where 149 patients received iopentol and 150 patients iohexol. There were no significant differences between the patients receiving the two contrast media with regard to demographic parameters, rate of injection or total dose of injected contrast media. No changes in blood pressure and no clinically important changes in heart rate were detected in the two groups. No serious adverse effects occurred. Seven patients (5%) in the iohexol and 12 patients (8%) in the iopentol group experienced adverse effects other than a sensation of warmth. Fourteen iohexol patients (9%) and 18 iopentol patients (12%) experienced warmth related to the contrast injection. Excellent films were obtained in most patients and no difference in diagnostic quality between iopentol and iohexol was observed.