Arsenic was present in Napoleon's hair before he arrived on Saint Helena and the findings at necropsy are consistent only with the diagnosis of ulcerating, regionally invasive, gastric carcinoma. The question of whether Napoleon died of, or merely with, arsenic poisoning is illuminated by developments in the treatment of promyelocytic leukaemia. Arsenic trioxide induces remission in many, but treatment can be complicated by QT prolongation, torsades de pointes and sudden death. At clinically relevant concentrations, arsenic blocks both I(Kr) and I(ks) channels and, at the same time, activates I(K-ATP) channels. The balance of these forces is easily disrupted, and QT prolongation is worsened by hypokalaemia. Napoleon was chronically treated with tartar emetic for gastrointestinal symptoms, and the day before he died he was given a huge dose of calomel (mercurous chloride) as a purgative. Both treatments would have caused potassium wastage. In addition, the Emperor was being treated with a decoction containing 'bark'-presumably 'Jesuit's bark'. The quinine in Jesuit's bark is another cause of QT prolongation. It is likely that the immediate cause of the Emperor's death was torsades de pointes, brought on by chronic exposure to arsenic and a medication error.