Diagnosis of blastic plasmacytoid dendritic cell neoplasm (BPDCN) or plasmacytoid dendritic cell leukemia (pDCL) is mainly based on immunophenotypical characterization of leukemic cells in blood or bone marrow samples. We tested by flow cytometry intracellular expression of the proto-oncogene T-cell leukemia 1 (TCL1), as well as membrane and intracellular expression of immunoglobulin-like transcript 7 (ILT7) in 21 pDCL samples and 61 non-pDC acute leukemia samples [i.e., 14 B-acute lymphoblastic leukemia (B-ALL), 9 T-ALL and 38 acute myeloid leukemia (AML)]. TCL1 is highly expressed in all pDCL samples while at a statistically lower level in all B-ALL and 34% of AML. Statistical analysis shows that intensity of TCL1 expression is a good marker for differential diagnosis of pDCL versus other acute leukemia (area under the receiver-operating characteristic curve, [AUC]: 0.96). By contrast, ILT7 positivity is limited to few pDCL samples and cannot be useful for diagnosis purpose. In conclusion, high intracellular intensity of TCL1 expression is currently the best marker for pDC lineage assignment by flow cytometry, which is particularly useful to distinguish pDCL from CD4(+) CD56(+/-) undifferentiated or monoblastic acute leukemia. Thus, intracellular TCL1 detection should be included in acute leukemia diagnosis panels used in hematology laboratories. © 2012 International Society for Advancement of Cytometry.