Nineteen children received 99mTc-DTPA for renography. The next day they received a simultaneous injection of the non-ionic contrast medium iopentol for urography and another injection of 99mTc-DTPA. The glomerular filtration rate (GFR) was estimated from the plasma elimination of 99mTc-DTPA as well as iopentol. Serum concentrations of creatinine and beta 2-microglobulin, and urine concentrations of creatinine, beta 2-microglobulin, alkaline phosphatase, N-acetyl-glucosaminidase, and albumin were determined. A significant reduction (12 +/- 3%) of GFR was observed after the injection of iopentol, without a subsequent rise in serum creatinine or beta 2-microglobulin. The urinary excretion of albumin and beta 2-microglobulin remained unchanged, while the excretion of alkaline phosphatase and N-acetyl-glucosaminidase was significantly increased after the urography, indicating some tubular effects of iopentol. Iopentol caused few and mild adverse events, the diagnostic yield was high, and the small changes in the renal tubular function parameters are presumed to be without clinical importance. The observed depressive effect on the GFR demands further investigations before iopentol can be recommended as a GFR-marker in children.