AML-M0 is an infrequent form of acute myeloblastic leukemia characterized by negative reaction with myeloperoxidase (MPO), Sudan Black and lymphoid antigens and positivity for CD13 or CD33. In the present study we describe the immunophenotypical and ultrastructural characteristics of a group of AML-M0 in adult patients. Nine out 218 AML leukemias (4.1%) fulfilled the AML-M0 criteria. CD13 or CD33 were positive in eight out nine cases, with two or more positive myeloid antigens being present in 82% of the cases. Immunological MPO was positive in 57% of the cases and CD68 in 33%. In no case megakaryocytic and erythroid markers present. Four cases (44%) expressed CD7 and TdT but only two coexpressed both antigens. In none of the cases was CD3 or CD22 cytoplasmic expression found. Ultrastructurally, a low number of granules was seen in all cases whereas ferritin particles or rhopheocytosis were not observed. Ultrastructural MPO was positive in one out of five cases and platelet peroxidase (PPO) was negative in the four cases studied. Two out of six cases showed karyotypic abnormalities (hypotetraploidy and a complex karyotype, respectively). In two out three cases a rearranged pattern for JH gene was observed. TCR (Cbeta and Jgamma) rearrangements were not detected in any case. AML-M0 is an infrequent form of acute myeloblastic leukemia. A large panel of myeloid monoclonal antibodies (MoAb) and the study of the cytoplasmic expression of myeloid antigens is necessary to diagnose this form of leukemia. AML-M0 usually coexpress lymphoid markers. Ultrastructural studies may be of help to discard an immature erythroid proliferation.