Hyperleukocytosis is a typical presentation of chronic myelogenous leukemia (CML). It sometimes induces leukostasis, the symptoms of which include visual change, headache, tinnitus, dizziness, and occasional disturbance of consciousness. In the present study, a 26-year-old male patient visited a general physician, who observed marked hyperleukocytosis and referred the patient to our hospital. The patient was diagnosed with CML and treated with a tyrosine kinase inhibitor and hydroxycarbamide. On the fourth day after admission, the patient suddenly complained of left-sided hearing loss. An audiogram revealed profound left sensorineural hearing loss. Magnetic resonance imaging of the head showed no lesions in the inner ear, cerebellum, or brain stem; therefore, we diagnosed sudden hearing loss due to leukostasis. Subsequently, his hearing did not improve, despite a decrease in leukocytes. The pathophysiology of leukocytosis involves increased leukocytes and thrombi, which induce high blood viscosity in the microcirculation. Leukostasis-related infarction and hemorrhage can lead to occlusion of the labyrinthine artery, causing deafness. Physicians should be aware that deafness can develop when diagnosing marked leukocytosis because such deafness is irreversible in most cases
