A case of groin ganglion with asymptomatic compression of the femoral vein is described. A 2-year-old girl was referred because of a symptomless groin mass. A mass was palpable in the right femoral triangle. Computed tomography and ultrasonography revealed a cystic lesion compressing the femoral vein ventrally. Prompt surgical removal of the cystic lesion was done without complications. Histopathological examination showed a benign structure similar to that of a ganglion.