A 53-year-old male patient was admitted to our hospital with painful splenomegaly. He was diagnosed as having chronic myelomonocytic leukemia (CMML) with leukocytosis, monocytosis, increased lysozyme concentrations in serum und urine, and lack of the Philadelphia chromosome. The clinical course of the disease was characterized by rapidly rising leukocyte counts, cutaneous infiltrates, respiratory insufficiency and neurological symptoms. Excessive hyperleukocytosis with a significant increase in monocytic cells led to microcirculatory obstruction, vascular endothelial damage and organ malfunction. This complication could not be prevented by low-dose chemotherapy with cytosine arabinoside. The patient finally died from pulmonary and cerebral hyperleukocytic syndrome.