Aim To investigate the correlation between implant appearance on ultrasound (US) and voiding cystourethrography (VCUG) results after dextranomer–hyaluronic acid copolymer (DxHA) injection in children with vesicoureteral reflux (VUR). Materials and methods Consecutive cases of primary VUR treated by endoscopic subureteral injection of DxHA were retrospectively reviewed. All children had postoperative bladder US and VCUG with a mean interval of 34 days and 47 days after injection, respectively. VUR resolution at postoperative VCUG was considered as treatment success. Implant appearance on US was graded according to the retained volume and its location by visual inspection; it was then correlated with VCUG results using the Spearman correlation coefficient. Results A total of 36 children (56 ureters) were identified in which 38 ureters (68%) had a clearly visualized implant on postoperative US and 40 ureters (71%) showed VUR resolution. The sensitivity of implant visualization on US for predicting reflux resolution was 73% (29/40), specificity 44% (7/16), positive predictive value 76% (29/38), and negative predictive value 39% (7/18). The grade 1, grade 2, and grade 3 implant appearances showed VUR resolution in 88% (22/25), 54% (7/13), and 61% (11/18), and showed persistent VUR in 8% (2/25), 15% (2/13), and 28% (5/18), respectively (p = 0.032). Conclusion The implant appearance on postoperative US showed good correlation with VCUG results in the early post-injection period. Large retained implants were associated with treatment success, while small or non-visualized implants were related to the persistent reflux.