Acute Megakaryoblastic Leukaemia (AMKL - FAB M7) is a rare hematological neoplasm that affects megakaryocytes. It accounts for 1% to 2% of adult AML cases, with an extremely poor prognosis, resulting in an overall survival of less than one year. While it was classified as AML-M7 by the French-American-British Group in 1985, its diagnosis has become more accurate with the development and application of flow cytometry immunophenotyping. According to the current WHO criteria, the diagnosis is established with the presence of ≥20% blasts among nucleated cells in the bone marrow, with >50% of these being of megakaryocytic lineage, expressing at least one or more of the platelet glycoproteins: CD41 (glycoprotein IIb), CD61 (glycoprotein IIIa), or CD42b (glycoprotein Ib). Between 20% and 30% of patients often present splenomegaly and hepatomegaly, without lymphadenopathy, although superficial lymphadenopathy has been described. Patients exhibit cytopenias, frequently thrombocytopenia, though some develop thrombocytosis. Megakaryoblasts are usually medium to large blasts, with round or slightly irregular or indented nuclei, fine reticular chromatin, and 1 to 3 nucleoli. The cytoplasm is basophilic, often agranular, and may show distinct blebs or pseudopod formation. We report a case of a 22-year-old male patient with fever, prostration, asthenia, dry cough, and abdominal pain radiating to the chest. Physical examination revealed splenomegaly and inguinal lymphadenopathy. The initial suspicion was primary mediastinal lymphoma due to chest imaging abnormalities. The patient had significant alterations in lactate dehydrogenase (LDH) and in complete blood count. The bone marrow was hypercellular, with hypoplasia of the erythroblastic, granulocytic, and megakaryoblastic series, and with the presence of 58.5% medium-sized cells, a high nucleus-to-cytoplasmic (N/C) ratio, euchromatin with presence of, sometimes irregular, nucleoli. Cytogenetic analysis revealed a complex karyotype with various numerical and structural alterations. BCR::ABL1 fusion research was negative. Immunophenotyping analysis showed large and medium-sized blasts with complex cytoplasmic characteristics, expressing myeloid phenotypic markers, and aberrant expression of CD7 and CD56 antigens, as well as CD41 and CD61. The key to diagnosing rare and challenging cases such as adult AMKL lies in correlating patient clinical information with robust laboratory test results that require in-depth knowledge, such as flow cytometry immunophenotyping. Furthermore, in these cases, physicians should have access to these test results as quickly as possible to make timely and accurate therapeutic decisions.