A randomized clinical trial was conducted to determine the optimum does of iohexol (iodine concentration 300 mgI/ml) as a contrast agent for excretory urography in screening outpatients. Cases were divided into two groups, 30 cases in Group A receiving 0.4 ml/kg of iohexol and 29 cases in group B receiving 0.8 ml/kg. Urography was performed at intervals of 5 minutes and 10 minutes following the injection. The diagnostic effectiveness of various X-ray images including nephrograms, the pyelocaliceal system, ureter and bladder were assessed. There was a significant disparity between the two groups during pyelocaliceal urography after 5 minutes, with a fair diagnosis attainable for 73.3% of group A versus 89.6% for group B (P less than 0.05), but after 10 minutes the rates for the two groups had evened out to 93.4% for group A and 96.6% for group B. During urography of the ureter after 5 minutes, only 16.6% of the cases in group A could be easily diagnosed against 51.7% of the cases in group B (P less than 0.05). However, after 10 minutes the rate for group A was up to 56.7% while the rate for group B was 82.8% (P less than 0.1). In the ureter after 5 minutes, non-diagnosable images constituted 26.7% of group A and 10.4% of group B, while after 10 minutes this had dropped to 16.6% for group A and 0% for group B. A comprehensive study of these results suggests that for diagnostic screenings which use iohexol as a contrast agent in excretory urography, optimal results may be obtained using iohexol in amounts of between 0.4 ml/kg and 0.8 ml/kg.