Contralateral exploration in children with unilateral inguinal hernia is controversial. This study was done to identify risk factors for the development of contralateral inguinal hernia in patients with unilateral inguinal hernia. The clinical experience of 4,206 inguinal hernias repaired by one pediatric surgeon on 3,358 children at HanYang University Hospital from September 1979 to December 2002 was analyzed. 1,868 (55.6%) hernias occurred on right side, 1,190 (35.4%) on left side, and 300 (8.9%) were bilateral. 2,702 children were boys and 656 were girls (M:F=4.1:1). 170 children of 3,058 children with unilateral hernias (5.6%) developed contralateral inguinal hernia at 1 day to 95 months after herniotomy. 146 children were boys and 24 were girls (M:F=6.1:1). The patients who had had herniotomy before 1 year of age developed contralateral hernia in 17.4%, compared with 5.6% overall average. The earlier the first herniotomy was performed, the more frequently contralateral hernia developed. The occurrence of contralateral inguinal hernia was more frequent in boys (146 of 2,460, 5.9%) than girls (24 of 598, 4.0%) and more frequent after left herniotomy (80 of 1,190, 6.7%) than after right herniotomy (90 of 1,868, 4.8%), but statistically not significant. 52.9% of contralateral inguinal hernia developed within 1 year after hernia repair, and 87.6% developed within 3 years. Routine exploration on contralateral side in children is not necessary.