A 51-year-old woman visited the emergency department because of intermittent fever for more than 2 weeks. Physical examination found symmetrical pitting edema of the extremities. Computed tomography and ultrasound showed multiple serous effusions and splenomegaly. A complete blood cell analysis showed moderate anemia and severe thrombocytopenia. Abnormal cells in a subsequent peripheral blood smear accounted for 5%. The bone marrow smear showed a large number of abnormal cells (69.5%), with large cell bodies and pseudopod protrusions. Huge multinucleated cells were visible. Nucleoli were visible in some nuclei, and mitotic figures were evident. The abnormal cells by peroxidase staining were negative, and the positive rate by periodic acid–Schiff staining was 62%. Bone marrow biopsy revealed diffuse infiltration of heterotypic cells (approximately 80%). Later, she was transferred to the hematology department. The phenotypes by flow cytometry were CD117bri+, CD30+, MPO−, CD2−, and CD25−. The molecular test for KIT gene mutation was negative, and multiple karyotypes were abnormal and complex. The final diagnosis was mast cell leukemia. The patient eventually died approximately 1 month after the diagnosis.