Inguinal hernia is the most frequent problem requiring surgery in children. Moreover, subsequent contralateral occurrence after repair of the symptomatic unilateral inguinal hernia(UIH) is not rare. This study is to evaluate the diagnostic value of inguinal ultrasonography (IUS) for potential bilateral inguinal hernia(BIH). A prospective study was performed for preschool children less than 6 years of age who were diagnosed as UIH from July 1999 to December 2000. We selected 58 cases with potential BIH, based on the past history, such as prematurity, ventriculo-peritoneal shunt, family history of BIH, hernia on the left side (LIH), age below 2, female, and contralateral positive silk glove sign on the physical examination. Screening with IUS and bilateral surgical exploration were applied on these cases. Forty-seven cases were males (81.0%) and 11 cases were females(19.0%). Thirty-four were infants. Symptomatic right inguinal hernia (RIH) were 28 (48.3%), and LIH were 30 cases (51.7%). Six cases had no evidence of contralateral patent process vaginalis (PPV) by IUS but showed contralateral PPV by operation, Two cases were suspicious to contralateral PPV under IUS, but operative findings were negative. Fifty cases showed contralateral PPV by IUS as well as operation. The detection rate of contralateral PPV under IUS was 86.2%. The preoperative IUS may reduce contralateral exploration.