We have identified ten patients with acute myeloid leukemia (AML) and one patient with chronic myeloid leukemia with megakaryocytic crisis who displayed an inv(3)(q21q26). Seven of them had an additional monosomy 7. Most of them had a myelodysplastic syndrome (MDS) preceding AML, normal or increased platelet counts, increased number of megakaryocyte, megakaryocytic dysplasia, and erythroid dysplasia. There was a high incidence of resistance to induction chemotherapy, short remission time, and early relapse. Seven patients were immunologically analyzed. The main immunophenotypes were as follow: CD7+, CD34+, HLA-DR+, CD38+, CD13+, CD33+, CDw65+, CD2-, CD3-, CD4-, CD8-, CD19+, CD20-, CD11b-. Our results suggest that the leukemia with inv(3)(q21q26) represents a new cytogenetic-clinicopathologic subtype, characterized by 1) abnormal megakaryopoiesis and multiple hematopoietic lineage involvement; 2) an antecedent MDS; 3) poor response to conventional chemotherapy; and 4) expression of CD7, CD34, CD38, HLA-DR, CD13, and CD33 antigens. We propose that the malignant transformation in patients with inv(3)(q21q26) occurs in an early stem cell prior to lineage commitment.