Children, especially females, very rarely present with acute urinary retention to the Emergency Department. In this case report, we present a case of urinary retention secondary to imperforate hymen leading to haematocolpos and mechanical obstruction of the urinary tract in a 12 year old adolescent girl. Imperforate hymen can be missed in adolescent girls presenting to Emergency Departments with urinary difficulty if genital examination or Emergency Ultrasound scan is not performed. There should be increased awareness among emergency physicians that imperforate hymen can be a possible cause urinary retention and lower abdominal pain in adolescent girls. We discuss the need of careful assessment along with a brief review of literature.