We investigated the safety and clinical effects of a new biocompatible, biodegradable treatment, the Deflux system (dextranomer microspheres in sodium hyaluronan solution), for the endoscopic treatment of grades III and IV vesicoureteral reflux. In preclinical safety studies in pigs histopathological examination demonstrated excellent tolerance. Two weeks after submucous implantation in the pig bladder early ingrowth of fibroblasts and recently generated collagen were noted at the implantation sites. At 14 weeks of followup this ingrowth had slightly increased. Long-term followup in rats showed that the volume of subcutaneous implants was slightly reduced (23%) 1 year after implantation. In a clinical study we investigated the implantation technique and the short (3 months) and long-term (1 year) effects of Deflux implantation in 75 children (101 ureters) with grades III and IV vesicoureteral reflux. We report data from up to 3 months of followup. Implant volumes of 0.4 to 1.0 ml. were sufficient to create distinct boluses and crescent-like ureteral orifices. Although viscous, due to its viscoelastic properties the substance was easy to inject in a well controlled manner. At cystography 3 months later reflux had resolved in 68% of implants, was reduced to grades I and II in 13% of implants and was unchanged in 19% of treated ureters (grades III and IV reflux). No signs of ureteral obstruction or adverse reactions were noted. Results from 1 year of followup will be reported later. Our results indicate that the dextranomer microspheres act as micro-carriers that promote ingrowth of fibroblasts and generate new collagen. We conclude that the Deflux system may represent a new, safe, simple alternative to endoscopic treatment of vesicoureteral reflux in children.