Aggressive natural killer-cell leukemia (ANKL) is a rare, lethal disease with pathologic features that are underdescribed in the literature, particularly in Western nations. In addition, although data on the molecular pathogenesis of ANKL has been reported, evaluation of such data in a clinicopathologic context remains limited. Patients diagnosed with ANKL were identified retrospectively. Detailed demographic and clinicopathologic data were analyzed. We assessed novel markers by immunohistochemistry and performed targeted next-generation sequencing analysis. The study group included 9 men and 3 women with a median age at diagnosis of 47.5 years (range, 20 to 75 y). Two distinct patterns of bone marrow involvement were identified: interstitial and sinusoidal. The neoplastic cells were positive for CD56 and CD94, and negative for surface CD3, CD5, and CD57 in all cases assessed. They were also positive for CD2 (10/12), c-MYC (6/8), BCL2 (6/8), CD16 (5/7), EBER (9/12), CD7 (6/11), pSTAT3 (3/8), CD8 (2/6), PD-L1 (2/8), CD4 (2/11), CD8 (2/6), and CD158 (1/5). Aberrant p53 expression was identified in most (7/8) cases; p53 was strongly expressed in 4 cases. Conventional cytogenetic analysis showed clonal abnormalities in 5 of 12 cases. TP53 mutations were detected in 3 of 6 cases, whereas ASXL1 and TET2 mutations were each detected in 2 of 6 cases. Patients had very poor outcomes despite intensive chemotherapy, with a median survival of 2 months. ANKL exhibits 2 distinct patterns of tissue involvement. Neoplastic cells in ANKL are commonly positive for c-MYC and EBER, and they have a high frequency of p53 overexpression, frequently with corresponding TP53 mutations.