A study was undertaken to determine the advisability of bilateral operation in all children with unilateral inguinal hernias. In a group of patients who had repair of a unilateral hernia, the incidence of a subsequent contralateral hernia was 12 per cent. In another group who presented clinically unilateral hernias and were subjected to exploration of the opposite side, the incidence of "positive" exploration was 56 per cent. The discrepancy between the two groups is discussed. From this study and the philosophy based on the concept of expected incidence of eventual bilaterality, the authors do not recommend routine exploration of the contralateral side.