PURPOSE: In the experimental model glutaraldehyde cross-linked collagen GAX 65 with a collagen concentration of 65 mg./ml. has proved to have more persistent implant volume and, therefore, a better antireflux effect than GAX 35, which is injected more commonly. The aim of this study was to evaluate the potential clinical application of GAX 65. MATERIALS AND METHODS: Ten boys and 8 girls an average of 4.6 years old presented with unilateral or bilateral primary reflux and were randomly divided into 2 groups. All refluxing ureters received 1 endoscopic subureteral injection of an average of approximately 2.9 ml. collagen. In group 1, 16 refluxing ureters were treated with GAX 65 and in group 2, 12 were treated with GAX 35. In all patients voiding cystourethrography was performed immediately after injection and 3 months postoperatively, and ultrasound was done on day 1, and at 1 and 3 months. Implant volume was calculated at the day of injection and at 3 months. The distribution of reflux grades was comparable in the 2 groups. RESULTS: Both materials had excellent injection properties. Immediately after injection all ureters were reflux-free. Of the 16 ureters treated with GAX 65 14 (87.5%) were reflux-free at the 3-month followup. Three months postoperatively ultrasound revealed an average implant volume increase of one-third. However, of the 12 ureters treated with GAX 35 reflux had resolved in 59.3% and the average implant volume had decreased by half 3 months after injection. CONCLUSIONS: Three months after endoscopic subureteral injection of GAX 65 vesicoureteral reflux was absent in 87.5% of patients. Therefore, GAX 65 has greater potential for treating reflux in the short term than GAX 35. To define the final efficacy of GAX 65 for treating vesicoureteral reflux, long-term evaluation of this series is necessary. In addition, further studies are mandatory to prove the long-term advantage of this new injectable substance.