Patients included in this trial should have been diagnosed to have Acute Promyelocytic Leukemia morphologically on FAB criteria. This is sufficient to initiate therapy with arsenic trioxide (ATO) but this diagnosis has to be confirmed using either Fluorescent in situ hybridization (FISH) for t(15;17) or reverse transcriptase polymerase chain reaction assay (RT-PCR) for PML-RARalpha transcripts within 7 days of inclusion into the study.