Naevocytosis is not necessarily leukemia; naevocytosis can be seen in acute leukemia, chronic leukemia, and leukemia-like reaction disorders. Leukemia-like reactions can be caused by infections or other stressful conditions. Once these conditions occur, patients are advised to undergo further tests to clarify the diagnosis, such as consulting a hematology clinic as soon as possible for history taking, physical examination, and further hematology and bone marrow examinations. Since naive cells are produced from bone marrow, bone marrow examination is inevitable and can identify not only benign and malignant diseases, but also the specific type. In addition, imaging may be required to find the cause of the patient’s infantile cells, such as a leukemia-like reaction in patients with severe lung infections. If a clear focus of infection is found on imaging, this can be combined with a bone marrow examination to be able to make further determinations.