Background Myelomonocytic acute myeloid leukemia (M4-AML) is frequently associated with the cytogenetic marker inv(16) and/or the presence of eosinophilia. The aim of this study was to analyze the incidence and prognostic role of these factors in a large series of patients. Design and Methods Adult patients with acute myeloid leukemia consecutively enrolled in the GIMEMA trials AML10 and LAM99p were retrospectively analyzed. Results Among 1686 patients, 400 cases of M4-AML were identified; of these, 78% had neither eosinophilia nor inv(16), 6% had eosinophilia only, 8% had inv(16) only and 8% had both. Univariate analysis showed that both eosinophilia and inv(16) were correlated with a higher probability of complete remission, lower resistance to chemotherapy and increased overall survival. Multivariate analysis showed that the simultaneous presence of the two factor; significantly increased the probabilities of both complete remission and overall survival. The presence of only one of the two factors also increased the probabilities of complete remission and overall survival, but not to a statistically significant extent. The relapse-free survival of the responding patients was not influenced by the two factors. Conclusions In a large series of patients with M4-AML we confirmed the favorable role of inv(16), but the weight of this factor among the whole M4 population was of limited relevance. Eosinophilia, which affects a small proportion of cases, also emerged as a favorable prognostic factor. Based on the results of this large case population, overall and relapse-free survival rates of patients with M4-AML are not significantly better than those of patients with non-M4 AML, while the concomitant presence of both inv(16) and eosinophilia was associated with a significantly improved prognosis.