We compared the results of treatment of identical injuries to the anterior superior iliac spine in two adolescent sisters, one treated conservatively and the other operatively. The functional results of both treatments were good, but the rehabilitation period was shorter with operative treatment, allowing an earlier return to athletic activity. Treatment by open reduction and internal fixation should be considered in patients requiring a short convalescence. Familial coincidence of avulsion fracture of the anterior superior iliac spine has not previously been described in the literature.