BACKGROUND Various factors have been implicated in the pathogenesis of infarction after posterior fossa surgery such as venous air embolism, patient's position (seated or prone), hyperflexion of the neck, excessive spinal cord traction, cervical canal stenosis, and systemic arterial hypotension. The main aim of this case report was to elucidate a case in which hydrogen peroxide was implicated in a major and systemic complication after a neurosurgical procedure. CASE REPORT We describe the case of a 5-year-old female patient who was admitted to our hospital because of a cerebellar hemispheric astrocytoma associated with obstructive hydrocephalus and accompanied by 2 syringomyelic cavities in the cervicothoracic portion of the spinal cord. Immediately after gross total resection of the lesion, impaired mobility of the upper and lower extremities was observed, a finding that was not consistent with intraoperative neurophysiologic monitoring data. Hydrogen peroxide had been judiciously used to irrigate the resection tumor cavity. In the next few postoperative days, the patient suffered from transient diabetes insipidus and hyperpyrexia, indicative of hypothalamic injury. CONCLUSIONS Neurological evaluation of the patient, after stabilization of her medical condition, revealed residual spasticity of upper and lower extremities, rendering her able to mobilize via the aid of wheelchair only. The most possible pathophysiologic explanation of her neurological deterioration, including hypothalamic dysfunction, was analyzed. The role of hydrogen peroxide as a source of free radical formation, and its co-responsibility for vascular platelet aggregation and vasoconstriction was considered, upon case review, the main responsible etiologic factor.