Gliomatosis cerebri is a rare form of diffusely infiltrating glioma that is typically resistant to conventional chemotherapy and radiation therapy and carries a poor prognosis. Temozolomide has shown antineoplastic activity against malignant gliomas and more recently was beneficial in one patient with gliomatosis cerebri. To make an objective assessment of the effect of long-term temozolomide administration in a patient with gliomatosis cerebri we used brain proton magnetic resonance spectroscopy and structural MRI. A 46-year-old man with gliomatosis cerebri was treated with temozolomide (200 mg/m(2) per day for 5 days every 28 days). Twenty cycles of temozolomide resulted in a marked reduction in choline and scyllo-inositol content, as detected using brain proton MR spectroscopy, indicating reduced tumor cellularity and/or growth rate. Neurochemical improvements were associated with normalization of the signal intensity in most of the previously affected cerebral regions and regression of mass effect on MRI. A left pyramidal syndrome, present at the start of the treatment, disappeared. Our observation lends support to larger clinical trials evaluating the use of temozolomide to treat this brain tumor.