In 1999, we reported seven cases of an unusual hematologic malignancy with primary cutaneous presentation that appeared as a distinct clinicopathologic entity characterized by medium-sized tumor cells with a peculiar CD3- CD4+ CD56+ CD43+ HLA-DR+ cell surface phenotype. Because the origin of tumor cells was not clear and they exhibited a nonlineage-specific phenotype, we hypothesized that such tumors likely originated from hematologic-myeloid precursor cells and were tentatively assigned the designation "agranular CD4+ CD56+ hematodermic neoplasms." In the present study we report 14 cases (seven already reported and seven additional cases) of these tumors, and simultaneously we present now a rare population of cells that we have identified in the peripheral blood of healthy volunteers treated with Flt3 ligand. These cells express all the characteristic markers of CD4+ CD56+ hematodermic neoplasms. This population appears to be related to plasmacytoid monocytes because they also expressed CD68 and bright levels of CD123. To confirm the relationship between these normal cells and CD4+ CD56+ hematodermic neoplasms, we conducted an extensive comparative phenotypic study. Results show that these two cell types are indeed related because they share many phenotypic features, including the presence of CD4, CD56, CD43, CD68, and HLA-DR and the absence of other T, B, NK, or myelomonocytic markers. More importantly, we found that the bright expression of CD123 by immunohistochemistry is a distinctive characteristic of CD4+ CD56+ hematodermic neoplasms because all (n = 14) cases expressed this marker, whereas only two specimens in a control panel comprising 30 samples of related tumors expressed comparable levels of CD123. We therefore propose that oncogenic transformation of NCAM-expressing plasmacytoid monocyte-like cells may lead to "agranular CD4+ CD56+ hematodermic neoplasm."