The patient was a 57-year-old male. He did not have any symptoms, but he had been noted one year previously to have a spleen mass whose diameter was 41 mm. It was followed up because most spleen tumors were benign. The patient was admitted to hospital and underwent tooth extraction and cyst fenestration for a left odontogenic cyst. On the day after the surgery, he had a fever and complained of abdominal pain, and an abdominal contrast-enhanced computed tomography (CT) scan revealed acute cholecystitis with gallstones. At that time, an 85-mm mass was incidentally found in his spleen, and it had grown larger than before. Laboratory examination showed that his hemoglobin was 12.5 g/L, total leukocyte count was 5.2 × 109/L, platelet count was 328 × 109/L, and soluble interleukin-2 receptor was 322 U/mL. Abdominal contrast-enhanced CT and magnetic resonance imaging (MRI) showed a huge, irregularly shaped, and heterogeneously enhanced mass in the spleen, with findings of suspected involvement of the transverse colon, left kidney, pancreatic tail, and left diaphragm (Fig. 1a–e). Positron emission tomography (PET)–CT showed an accumulation of standardized uptake value (SUV) max of 4.1 and no obvious lymph node or distant metastasis (Fig. 2a, b). Based on the results of these imaging studies, malignant lymphoma (ML) or some kind of sarcoma was presented as the differential diagnosis. Colonoscopy showed an easily bleeding lesion with mucosal irregularities in the splenic flexure. A biopsy of the intestinal mucosa was performed at that time, and a histological examination suspected MS invading to the colon. Since the tumor was huge and had invaded the surrounding organs, especially the colon, and since it was most important to confirm the diagnosis, we decided to perform surgery. After informed consent by the patient, the surgical treatment followed; a total splenectomy, partial pancreatectomy, partial colectomy, left nephrectomy, left diaphragm partial resection, and cholecystectomy were performed (Fig. 3a–c). Postoperative histological examination revealed a diagnosis of MS in the primary of the spleen with presence of irregular karyotype cells, with immunohistochemical diffusely strong positivity for CD33 (Fig. 4), partially positivity for CD68, but negativity for CD3, CD20, CD34, CD56, myeloperoxidase (MPO), and naphthol AS-D chloroacetate esterase staining, and with a chromosomal test. The chromosome test showed an abnormal karyotype of 45, XY, del (1) (p13p22), add (6) (q13), add (10) (q22), -15, add (16) (q24)/ 46, XY, add (1) (q21), add (4) (q21), add (6) (q13), add (10) (q22), add (16) (q24), which indicated the complex karyotype, a poor prognostic type of AML. Bone marrow biopsy showed no abnormal cells, and ruled out leukocytosis. Postoperative complications included an ileus and intra-abdominal abscess on postoperative day 3, which was treated with drainage management and antibiotics, and finally, he was discharged home on postoperative day 62. After the surgery, he discussed the treatment strategy with hematologists, and it was decided that he would undergo bone marrow trephine biopsy. Immunophenotypic flow cytometry revealed no evidence of myeloid leukemia. He underwent umbilical cord blood transplantation, and he is now living without a sign of recurrence at 10 months after surgery.