While the t(8;21) translocation is one of the most recurrent chromosomal abnormalities in acute myeloid leukemia, prognostic studies have been hampered by the relatively few number of patients reported. We thus performed a large retrospective study in 161 adults and children with t(8;21) acute myeloid leukemia, all prospectively enrolled in 6 different trials conducted in France between 1987 and 1998 (median follow-up 4.9 years). Prognostic studies were performed in the 154 patients who achieved a complete remission. Individual data were registered, including sex, age, blood and marrow counts, extramedullary disease, and cytogenetics. The value of allogeneic stem cell transplantation versus chemotherapy as postremission therapy was evaluated according to the intent-to-treat principle. Estimated 5-year disease-free survival (DFS) and overall survival were 52% and 59%, respectively. Outcome was not significantly better in patients from the stem cell transplantation group (estimated 5-year DFS and survival, 56% vs 52% and 67% vs 57%; P =.55 and.64, respectively). White blood cell count (WBC) was the only identified prognostic factor. To further take into account the spontaneous differentiation potential of the leukemic clone, a WBC index was derived as the product of WBC by the ratio of marrow blast. This WBC index was a more powerful factor than the original WBC, allowing us to distinguish 3 subgroups of patients with different outcomes (low index, < 2.5; intermediate index, 2.5-20; high index, 20 or more). In multivariate analysis, the WBC index was the only prognostic factor for DFS (P =.003), complete remission duration (P =.002), and overall survival (P =.04).