The transcription factor C/EBPalpha is crucial for differentiation of mature granulocytes. Recently, different CEBPA gene mutations likely to induce differentiation arrest have been described in nearly 10% of patients with acute myeloid leukemia (AML). In the present study, we retrospectively analyzed the prognostic significance of CEBPA mutations in 135 AML patients (French-American-British [FAB]-M3 excluded). All patients were prospectively enrolled between 1990 and 1996 in a multicenter trial of the ALFA (Acute Leukemia French Association) Group (median age 45 years, median follow-up 5.7 years). Mutations were assessed using direct sequencing of the CEBPA gene. Twenty-two mutations were found in 15 (11%) of 135 patients tested. Twelve patients had at least one mutation located in the N-terminal part of the protein leading to the lack of expression of the full-length C/EBPalpha protein. CEBPA mutations were present only in patients belonging to the intermediate cytogenetic risk subgroup and associated with the FAB-M1 subtype (P =.02). FLT3 internal tandem duplication (ITD) was found in 5 of 15 CEBPA-mutated as compared with 30 of 119 CEBPA-nonmutated cases tested (P =.54). Presence of CEBPA mutations was identified as an independent good prognosis factor for outcome even after adjustment on cytogenetics and FLT3 status (estimated 5-year overall survival 53% vs 25%, P =.04). FLT3-ITD appeared to act as a major bad prognosis factor in patients with CEBPA-mutated AML. We thus propose a risk classification that includes in the favorable subgroup all patients from the intermediate subgroup displaying CEBPA mutations when not associated with FLT3-ITD.