Idiopathic hypereosinophilic syndrome (HES) is a rare disorder marked by a sustained overproduction of eosinophils and a predilection for damage to multiple organ systems. Its neurologic involvement ranges from the central to the peripheral nervous system, and can be associated with eosinophilic myositis. We report a 68-year-old woman who had eosinophilia, eosinophilic dermatitis and eosinophilic pneumonia. She also suffered from numbness and weakness of the lower limbs. Because of long-lasting (> 6 mo) eosinophilia (> 1.5 x 10(9)/L) in the peripheral blood and the fact that no other underlying causes of eosinophilia and neurologic involvement could be identified, a diagnosis of idiopathic hypereosinophilic syndrome was made. The muscle biopsy showed infiltration of inflammatory cells, including a few eosinophils (Liu's stain). Magnetic resonance images, motor evoked potentials, somatosensory evoked potentials and nerve conduction velocities also showed abnormalities in the central and peripheral nervous systems. The pathogenesis and treatments of HES are discussed in this report.