A history of unexplained low back pain associated with chronic constipation in an adolescent girl of menarchal age or in an obviously postmenarchal girl should make one consider an imperforate hymen with hematocolpos. This is a particular important differential diagnosis in the work-up of an adolescent who denies ever having had menses or sexual activity. The case of a girl with an imperforate hymen presenting with a six-month history of chronic constipation and intermittent low back pain is described.