We reported two MDS/MPN-U and one CMML patients with CSF3R T618I mutation. There were two males and one female, with a median age of 84 years. Three patients presented with leukocytosis and anemia, two with thrombocytopenia, and one with monocytosis. In all the patients, bone marrow showed hypercellularity with myeloid or erythroid predominant trilineage hematopoiesis and dysplasia. Two cases carried -7/-7q abnormalities. In addition to CSF3R T618/I mutation, each case carried 3-5 additional somatic mutations. The median survival was only 2 months. These rare patients were characterized by old age, high mutation rates, clonal hematopoiesis-associated mutations, clonal evolution, and unfavorable prognosis.