We designed a phase II trial of arsenic trioxide (AT) for the treatment of relapsed and refractory acute lymphoblastic leukemia (ALL). The dose administered was 0.25 mg/kg/day intravenously for 5-7 days per week for up to 60 days. Of 11 patients eligible, eight had B-cell and three T-cell ALL and two were Philadelphia chromosome-positive. The median duration of therapy was 21 days (range 7-28). One patient died of an infection. There were no responses. Ten patients have died. The median survival was 3.2 months (range 1.2-4.1). We conclude that AT is not active in the treatment of ALL.