Three-dimensional computed tomography venography is a useful tool to identify increased saphenous vein diameter and provides a complementary road map for surgery in patients with varicose veins. In this study, we investigated the correlation between saphenous vein diameter on computed tomography venography and venous reflux detected on duplex ultraonography. We enrolled 152 patients (213 extremities) who underwent endovenous laser ablation therapy, following high ligation of the saphenofemoral junction between January 2014 and December 2019. All patients underwent preoperative computed tomography venography evaluation. The saphenous vein diameter was measured on computed tomography venography, and venous reflux was evaluated in the operating room using Doppler ultrasonography. Among the 152 patients included in the study, 61 showed varicose veins affecting the bilateral extremities. Among the 213 extremities investigated, 165 (77.5%) and 48 (22.5%) extremities showed varicosities involving the greater and lesser saphenous veins, respectively. Among all extremities, venous reflux was detected in 172 (80.8%). The mean diameter of the greater saphenous vein measured 5 cm distal to the saphenofemoral junction was 8.07±1.82 mm in patients with reflux and 5.11±1.20 mm in patients without reflux (p < .05). The small saphenous vein diameter measured 5 cm distal to the saphenopopliteal junction was 7.65±1.74 mm in patients with reflux and 5.04±1.80 mm in patients without reflux (p < .05). Based on the receiver operating characteristic curve, the greater saphenous vein threshold diameter of 5.880 mm measured 5 cm distal to the saphenofemoral junction was the optimal cut-off value to predict reflux (sensitivity 91.4%, specificity 81.8%). The lesser saphenous vein diameter of 5.285 mm measured 5 cm distal to the saphenopopliteal junction was the optimal cut-off value to predict reflux (sensitivity 94.9%, specificity 75.0%). Vein diameter cannot be used as an absolute reference for venous reflux; however, it may have predictive value in patients with varicose veins. Computed tomography venography based measurements of vein diameter may serve as a useful diagnostic tool to predict venous reflux and recommend treatment.