The characteristic reciprocal translocation t(15;17) of acute promyelocytic leukemia (APL) disrupts the PML gene on chromosome 15 and the retinoic acid receptor-alpha (RAR-alpha) gene on chromosome 17. PML/RAR-alpha fusion mRNAs are then transcribed and can be detected by a newly described reverse transcription polymerase chain reaction (RT-PCR) assay. Using RT followed by nested PCR amplification for PML/RAR-alpha, we serially evaluated bone marrow aspirates from patients with APL who were treated with all-trans retinoic acid (RA) for induction, followed by all-trans RA as maintenance or cytotoxic drugs as consolidation. At diagnosis, PML/RAR-alpha mRNA was detected in all patients. After initial therapy with all-trans RA, the RT-PCR assay remained positive after induction of complete remission in 31 of 32 evaluable patients. Maintenance treatment by all-trans RA alone was associated with persistent assay positivity and subsequent clinical relapse in 13 of 13 patients. By contrast, the test became negative in 19 of 20 newly diagnosed patients who received consolidation chemotherapy; the 1 patient who remained positive relapsed at 12 months. Three of the 19 assay-negative patients later converted to positive and subsequently relapsed; the remaining 16 patients have remained RT-PCR negative in sustained first remission, with a median follow-up duration that exceeds 24 months (range, 12+ to 34+ months). Despite induction of complete remission in a high proportion of patients, all-trans RA rarely eradicates molecular evidence of disease in patients with APL; however, subsequent treatment with cytotoxic chemotherapy frequently converts the RT-PCR assay for PML/RAR-alpha to negative. Serial negative tests are associated with prolonged disease-free survival, whereas persistence of a positive test after treatment is highly correlated with subsequent relapse. This test identifies patients in remission at high risk for relapse who may benefit from additional antileukemic therapy.