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Clonal mature plasmacytoid dendritic cell proliferations (MPDCP) are a recently recognized entity in the WHO fifth edition, but their pathologic spectrum remains poorly characterized. Cases with extensive MPDCP in the bone marrow (BM) are rare and can exhibit overlapping features with blastic plasmacytoid dendritic cell neoplasm (BPDCN). We compared clinicopathologic and genomic features of 11 patients with myeloid neoplasm-associated MPDCP to 5 patients with secondary BPDCN arising from clonally-related myeloid neoplasms. MPDCP exhibited variable degrees of BM involvement (5% to 50%) and architectural patterns, were uniformly positive for CD123, CD4, TCF4, and IRF8, variably positive for TCL1 (7/11), CD5 (7/11), and CD7 (2/11), and negative for SOX4, CD56, and TdT. MPDCP skin involvement was rare (1/11), with no CNS involvement. MPDCP heralded myeloid disease progression in a subset of patients, with increased blasts or progression to AML (4/11). In contrast, secondary BPDCN was uniformly positive for SOX4 and TCL1, with frequent CD56 (4/5) and subset/weak TdT (3/4). All BPDCN patients had characteristic skin lesions, and a subset with CNS involvement (2/5). The underlying myeloid neoplasms associated with MPDCP or BPDCN were enriched in TET2, SRSF2, ASXL1, RUNX1, and RAS pathway mutations. While karyotypic abnormalities were uncommon in MPDCP, all BPDCN showed chromosomal structural abnormalities and copy number variants, including deletions of 3p, 9p, and 12p. Our findings expand the histopathologic, immunophenotypic, and genetic characterization of MPDCP, and highlight pathologic features that distinguish it from BPDCN. Utilization of SOX4 immunohistochemistry, combined with careful clinical and molecular correlation, can aid in resolving these diagnostic challenges.