Prophylactic contralateral exploration in unilateral inguinal hernia repair is still controversial. The purpose of this study is to analyze the contralateral incidence of hernia and to verify the necessity of the simultaneous contralateral exploration. Infants and children operated on for inguinal hernia or hydrocele at the Department of Pediatric Surgery of Asan Medical Center from January 1996 to December 2005 were analyzed retrospectively. A total of 383 patients (9.8 %) out of 3,925 patients underwent a simultaneous bilateral operation. A total of 222 patients (6.2 %) out of 3,542 patients underwent a secondary metachronous contralateral operation after primary unilateral inguinal hernia or hydrocele repair. Because simultaneous bilateral operation cases included true bilateral inguinal hernia or hydrocele, and unilateral hernia and simultaneous contralateral exploration, bilateral incidence of inguinal hernia and hydrocele could be maximally considered as 15.4% (605 patients). Therefore, the prophylactic contralateral exploration in unilateral inguinal hernia or hydrocele should be determined carefully in considering history and physical examination of the patients, and postoperative complications.