6504 Background: Refinement of prognostic groups using molecular genetic features of the disease may facilitate the design of future risk-adapted trials for APL. Previous analyses of the prognostic value of PML-RARα transcript levels and isoform type have generated conflicting results. We sought to determine the clinical significance of measuring PML-RARα transcript level at the time of diagnosis using real-time quantitative RT-PCR (RQ-PCR) of the long (L) and short (S) isoforms of the fusion gene. Methods: PML-RARα transcript levels were measured in pretreatment bone marrow (BM) and/or paired blood (B) samples of 139 patients (pts) with newly diagnosed APL registered to C-9710, the intergroup phase III randomized study of concurrent tretinoin and chemotherapy with or without arsenic trioxide as initial consolidation therapy. Transcript levels were expressed as the normalized quotient (NQ) of PML-RARα/ABL or PML-RARα/GAPDH. Correlations between pre-treatment NQ values and other pre-treatment characteristics including age, presenting WBC and platelet count, and L or S isoform were explored. Results: Pre-treatment NQ values using ABL vs. GAPDH showed significant correlation (p < 0.0001). Pre-treatment transcript levels in B and BM for 57 pts with S-form were significantly higher than 82 L-form patients (p = 0.02) when NQ was determined using GAPDH as the control gene (but not with ABL). With a median follow-up of 24.3 months for all patients, there have been 19 events. Median DFS for either isoform has not been reached, however, DFS was significantly shorter for patients with S-isoform compared to those with L-isoform (p = 0.017) with an estimated hazard ratio of 3.28. Using a multivariate proportional hazard model, no other significant relationships were found between DFS and NQ level at diagnosis, age, presenting WBC or platelet count. Conclusions: The S-isoform of the PML-RARa fusion gene was associated with shorter DFS in a subset of patients entered onto C-9710. If confirmed in a larger cohort of C-9710 pts, presence of the S-isoform should be considered as a high-risk feature in the design of future risk-adapted treatment strategies for APL. No significant financial relationships to disclose.