The presence of neutrophilic leukocytosis may underlie a wide variety of diseases. Some rare causes of neutrophilia might be chronic neutrophilic leukemia (CNL) and myelodysplastic/myeloproliferative neoplasm with neutrophilia (MDS with neutrophilia). Here we report a case of a 78-year-old woman who came to our ER due to severe leukocytosis and anemia on a routine check-up. The patient was asymptomatic and the last exams available showed a mild leukopenia and thrombocytopenia. The abdominal echography showed mild splenomegaly The patient underwent bone marrow (BM) examinations. One week later, the patient presented mental deterioration. The patient underwent a cranial CT and RMN that showed multiple lesions of 11 mm in the brain parenchyma, cerebellum and encephalic trunk. Another week later, the clinical presentations worsened: she was in a comatous state and feverish 40 °C unresponsive to steroid therapy. Autopsy showed a leukemic and hemorrhage infiltration in multiple organs and in the BM a cellularity of 100% represented by myeloid elements with a slowdown maturation with blasts 5%. According to WHO 2016 this case can be reported as an aCML, an MDS/MPN overlap syndrome that is difficult to differentiate from a CNL.