To compare the diagnostic performance of contrast-enhanced voiding ultrasonography (CEVUS) and direct radionuclide cystography (DRC) at physiological bladder filling volumes in pediatric patients with suspected vesicoureteral reflux (VUR). This prospective, comparative study included 22 children with 44 renal units referred for evaluation due to recurrent urinary tract infections (UTIs). Bladder function was assessed as normal through a four-hour voiding observation or uroflowmetry. All patients underwent both DRC and CEVUS on the same day, using a single catheterization. The investigations were performed by blinded radiologists. The primary outcome was VUR detection in relation to bladder filling volumes. Secondary outcomes included adverse effects and clinical outcomes in relation to radiological findings. DRC detected VUR in 72.7% of patients and 50.0% of renal units, while CEVUS detected VUR in 77.3% and 52.3% respectively. Among patients with paired bladder-filling data at first VUR detection, median bladder filling volume was 37.3% (IQR 26.7-82.4) of expected bladder capacity for DRC and 66.7% (IQR 52.9-87.3) for CEVUS (Wilcoxon signed-rank p = 0.158). At patient level, paired detection rates were comparable between modalities (exact McNemar p = 1.00). At the renal unit level, intermodality agreement was moderate (Cohen's κ = 0.59; 95% CI 0.35-0.83). DRC detected VUR in 4 renal units negative on CEVUS, whereas CEVUS detected VUR in 5 renal units negative on DRC.