The frequency of occurrence of bilateral inguinal hernia in children in the first seven years of life is the subject of this study. Ninety consecutive cases of inguinal hernia have been personally operated upon by us. All of these children have had bilateral exploration of the inguinal canals, and we are reporting upon the frequency with which an unsuspected inguinal hernia or a patent processus vaginalis is found in patients presenting with unilateral hernia. In this series of 90 cases, 10 of the patients were girls. Two were first seen with bilateral hernias, and eight with unilateral hernias. Of the eight, four had a right- and four a left-sided defect. At operation, three of these girls were found to have an unsuspected hernia on the opposite side. Thus, approximately 40% of the girls with unilateral hernia in this series subsequently were shown to have a hernia in the apparently uninvolved