BACKGROUND: Little is known about the morphological and clinical features of the minority of acute myeloid leukemias (AML) that carry the t(9;22)(q34;q11) translocation. MATERIALS AND METHODS: Cytologic, cytogenetic and clinical features were studied at diagnosis and during disease evolution in 11 patients presenting with de novo AML. Diagnoses according to the FAB criteria were AML-M2 (3 cases), AML-MO and AML-M4 (2 cases each), AML-M4eos, AML-M5, AML-M6, AML-M7 (1 case each). RESULTS: Immunophenotyping disclosed positivity for CD33 or CD13 and for the CD34 stem cell antigen in all cases tested. Lymphoid-associated markers (LM) were detected in 7/9 patients. In 5 cases the expression of at least 2 LM was seen. In addition, evidence of clonal rearrangement of the immunoglobulin (Ig) and/or T-cell receptor (TCR) genes was documented in 3/4 evaluable cases. The Ph chromosome was found as the sole change in 5/11 cases; the karyotype reverted to normal in 2/4 patients who achieved complete remission. Rearrangement in the M-bcr region was detected by Southern blotting in 2/7 cases. CONCLUSIONS: This infrequent cytogenetic subset of de novo AML appears to be characterized by heterogeneous cytologic features, with frequent expression of lymphoid markers, and by unfavorable prognosis. The combination of clinical, cytologic and cytogenetic studies is important in distinguishing de novo AML with the t(9;22) from chronic myelogenous leukemia blast crisis.