This is a case report of a 72-year-old male with a newly diagnosed prostate cancer, who was treated with androgen deprivation therapy and anti-androgen. The patient was diagnosed with congenital long QT syndrome (LQTS), and a markedly worsening of the QT-prolongation was observed, after the hormonal therapy was initiated. He ultimately suffered an initially resuscitated cardiac arrest but did not survive to discharge. Pharmaceutical treatments in patients with LQTS should always be handled with caution, and if a treatment with a known or probable QT-prolonging drug is initiated, a cardiologist should be consulted.