We report a new case of the unusual myeloproliferative syndrome chronic neutrophilic leukemia (CNL) that met all the criteria generally required for the diagnosis of this entity. The patient presented abnormalities in platelet function not previously reported that may explain the bleeding tendency observed in these patients. The study of neutrophil function suggested also defective mobility and intracellular bactericidal activity. The chromosomal study revealed original abnormalities consisting of multiple chromosomal ruptures and figures. The disease was controlled with busulfan. After 20 months, the patient died of sepsis. An autopsy was performed confirming the diagnosis and ruling out the existence of a cause of a leukemoid reaction, such as cancer or granulomatous disease.