Arsenic compounds have been utilised in medicine for over 2,000 years. Recently, arsenic trioxide has been shown to induce complete remission in up to 90% of patients with acute promyelocytic leukemia (APL), including those resistant to standard therapy. Randomised studies are currently investigating the potential clinical utility of this drug in APL. In vitro, arsenic trioxide exhibits antitumoral properties with respect to various other tumor species as well, although assessment of clinical effects will require further study. This article reviews clinical results and possible mechanisms of action of the drug.