Purpose: There are no diagnostic recommendations for the evaluation of men prior to therapeutic intervention for clinical varicoceles. The aim of this study was to define a simple and reliable hemodynamic parameter predicting the presence of reflux in clinical varicoceles (grade II/III) compared to the gold-standard of venography. Methods: Data were retrospectively collected on men presenting to a tertiary referral center with subfertility/infertility, testicular pain or palpable mass over an 11-year period (2004-2015). Men with clinical varicoceles (defined as grade II/III) underwent color doppler ultrasound (CDU) and maximum venous diameter (MVD) was measured and correlated to reflux at venography, at the time of therapeutic embolization. Receiver-operator characteristic analyses identified the threshold MVD to detect reflux. Results: A total of 107 men (70 unilateral, 37 bilateral) underwent CDU. From all, 144 testis units were included with a mean MVD of 3.5 mm (range 2 mm to 7 mm). CDU-reflux is demonstrated in 108 of 144 (75.0%) testes and 97 testes underwent venography of which 73 (75.3%) had demonstrable reflux. MVD in testes with venography-detected reflux was 4.0 mm (range 2 mm to 7 mm) compared to 2.7 mm (range 2 mm to 4 mm) in those without reflux (p<0.01). The optimal MVD for discriminating men with and without venography-diagnosed reflux was 3.0 mm (sensitivity-95.9%, specificity-58.3%, correctly-classified 86.6%, AUC=0.86). MVD = 3 mm was a more accurate discriminator for identifying patients with venography-reflux compared to CDUreflux alone (correctly-classified 82.5%, AUC=0.73). Conclusions: A threshold value MVD = 3 mm accurately identifies more men with proven venography-reflux than using CDU-detected reflux. MVD is a simple parameter that can be used to stratify men with clinical varicoceles, who may benefit from therapeutic intervention.