If even 50,000 US cases currently exist, this congenital or acquired disease occurs three times more often than acute lymphoblastic leukaemia (the most common childhood leukaemia), one third as often as cystic fibrosis, and twice as often as phenylketonuria. Large numbers of LQTS cases go undiagnosed due to the complexity of the diagnosis, the variety of presentations, and many clinicians' lack of familiarity with the disease.