The presence of an acutely inflamed vermiform appendix in a femoral hernia sac is extremely rare; the condition is termed De Garengeot's hernia. Here we describe an elderly patient for whom preoperative computed tomography aided the diagnosis of this rare entity. This Chinese woman had presented with a painful right groin mass. The patient successfully underwent an emergency appendicectomy and primary femoral hernia repair. Once diagnosed, it is imperative to follow key surgical principles to limit the spread of infection.