To determine the limits for the amount of tunical fluid enough to be termed as hydrocele by using extended-field of view US technology and to define hydrocele for the first time with standard numerical criteria.A total of 60 patients were evaluated in this prospective study. Group 1 consisted of 20 patients with unilateral clinical hydrocele. Group 2 included 20 patients referred for scrotal US for reasons other than hydrocele. Group 3 comprised 20 male controls with no scrotal complaints. Testis volume (V t ), scrotum volume (V s ) and V t /V s ratio were calculated for each subject by dimensions measured in longitudinal and tranverse planes.Mean V t /V s ratio was 0.28±0.17 (range, 0.07–0.57), 0.69±0.08 (range, 0.53–0.80) and 0.71±0.07 (range, 0.61–0.85) for groups 1, 2 and 3, respectively. Mean V t /V s for group 1 was significantly lower than those for groups 2 and 3, respectively (p<0.001 for each). Based on the ROC curve analysis, 0.55 for V t /V s ratio was determined as the optimal cut-off point below which the US diagnosis of hydrocele could be made (sensitivity 95.0% and specificity 97.5%).We propose a novel US parameter of V t /V s ratio below 0.55 for a standard distinction between hydrocele and physiological amount of scrotal fluid.