We report the case of a man in his 50s with refractory acute myelomonocytic leukaemia (AML) who presented with neck swelling, fever, and elevated levels of C-reactive protein (CPR) after venous punctures. An infected haematoma was presumed, but the patient showed no signs of improvement under broad-range antibiotics, and microbiological results were negative. The subsequent development of a rapidly evolving erythematous-violaceous plaque around a site that had previously punctured on the extensor surface of the right arm prompted us to reconsider the clinical setting as a whole and consider the hypothesis of deep neutrophilic dermatosis (ND) associated with haematologic malignancy. A biopsy of the arm lesion showed an aseptic neutrophilic infiltrate, confirming this diagnosis. The patient was initially treated with high-dose intravenous corticosteroids, resulting in a dramatic improvement of the skin lesions.