Avulsion fracture of ischial tuberosity which usually occurs in a young athelete is often undiagnosed early, and therefore is treated after a considerable time elapsed. And it causes a lot of discomfort to the athelete because of subsequent tumorous enlargement of tuberosity with fibrous union, and hamstring dysfunction as a late sequela. Two cases of this sort that required excision of the ununited enlarged fragment are presented. The present authors feel that widely displaced avulsion fracture of ischial tuberosity should be treated operatively and that even minimally displaced one also has to be protected from another injury.