Cytological examination of cerebrospinal fluid (CSF) is used not only for the diagnosis of spinal disease, but also to assess the postoperative effect of treatment. We experienced a case of high-grade glioma in disseminated CSF, and retrospectively examined the clinical, pathological and cytological features. We further investigated radiation-induced DNA damage in glioma cells using phospho-Histone H2AX antibody. A five-year-old boy received a clinical diagnosis of optic nerve glioma, and was followed-up for three months after chemotherapy. Magnetic resonance imaging was repeated, revealing abnormalities in other brain areas. The pathological diagnosis was anaplastic astrocytoma. CSF dissemination was detected, and increases in the number and mitosis of tumor cells were observed in CSF cytology. After radiotherapy the tumor cells in CSF decreased markedly. On cytomorphologic and immunocytochemical evaluation post-irradiation, tumor cells showed vacuolation of both the nucleus and cytoplasm, degeneration of nuclear chromatin, and alteration of the phospho-Histone H2AX expression, compared with tumor cells before the irradiation. CSF cytology is an effective means of evaluating DNA damage in tumor cells after irradiation, and may be useful in assessing the therapeutic response. Diagn. Cytopathol. 2011; © 2011 Wiley-Liss, Inc.