The biology and outcome of adult t(4;11)(q21;q23)/MLL-AFF1 acute lymphoblastic leukemia are poorly understood. We describe the outcome and delineate prognostic factors and optimal post-remission therapy in 85 consecutive patients (median age 38 years) treated uniformly in the prospective trial UKALLXII/ECOG2993. The immunophenotype of this leukemia was pro-B (CD10(NEG)). Immaturity was further suggested by high expression of the stem-cell antigens, CD133 and CD135, although CD34 expression was significantly lower than in t(4;11)-negative patients. Complete remission was achieved in 77 (93%) patients but only 35% survived 5 years (95% CI: 25-45%); the relapse rate was 45% (95% CI: 33-58%). Thirty-one patients underwent allogeneic transplantation in first remission (15 sibling donors and 16 unrelated donors): with 5-year survival rates of 56% and 67% respectively, only 2/31 patients relapsed. This compares with a 24% survival rate and 59% relapse rate in 46 patients who received post-remission chemotherapy. A major determinant of outcome was age with 71% of patients aged <25 years surviving. Younger patients had lower relapse rates (19%) but most received allografts in first complete remission. In conclusion, multivariate analysis did not demonstrate an advantage of allografting over chemotherapy but only five younger patients received chemotherapy. Prospective trials are required to determine whether poor outcomes in older patients can be improved by reduced-intensity conditioning allografts. NCT00002514 www.clinicaltrials.gov.