Purpose To determine if large caliber accessory veins are associated with radial-cephalic arteriovenous fistula (RC-AVF) non-maturation. Methods RC-AVFs were created in 15 consecutive patients (radial artery and cephalic vein diameter >2 mm, in the absence of arterial inflow or venous outflow stenoses or occlusions). Contrast-enhanced magnetic resonance angiography (CE-MRA) was performed preoperatively for the determination of vessel diameters, stenoses and occlusions. The location and caliber of accessory veins was determined. Vascular access (VA) function was monitored and all interventions required to obtain a functioning VA were recorded. Non-maturation was defined as a nonfunctional VA at 2 months after creation. The predictive value of accessory vein caliber for prediction of RC-AVF non-maturation was evaluated using receiver operating characteristic (ROC) analysis. Results Non-maturation occurred in 10 (67%) out of 15 RC-AVFs. Large caliber accessory veins (n=4), venous stenosis (n=3) or both (n=2) were associated with RC-AVF non-maturation. The presence of large caliber accessory veins was the only significant predictor for RC-AVF non-maturation (p=0.01). Preoperatively detected accessory veins with a diameter >70% of the cephalic vein diameter, had a sensitivity, specificity, positive predictive value (PPV) and negative predictive value (NPV) of 80, 100, 100 and 91% for prediction of RC-AVF non-maturation on patient level. Accessory vein ligation and dilatation of venous stenosis resulted in an overall salvage success rate of 89% (8/9). Conclusion Large caliber accessory veins are associated with RC-AVF non-maturation. Ligation of large caliber accessory veins is a successful salvage procedure in a substantial group of patients. Furthermore, ligation of these accessory veins during initial RC-AVF creation can potentially reduce non-maturation rates; and therefore, preoperative assessment of accessory veins is recommended.