In ten patients with malignant gliomas the regional cerebral metabolic rate for glucose (rCMRGlc) was studied with positron emission tomography (PET) using 2-18F-deoxyglucose (18FDG) before and within 1 to 7 days after intra-arterial chemotherapy with the nitrosourea derivative ACNU (iaACNU). Three patients were studied before and after two iaACNU courses and one patients before and after three iaACNU courses. The early effects of iaACNU on tumour rCMRGlc were highly variable and appeared to be more pronounced after the first course of iaACNU than in later iaACNU courses, i.e. more pronounced in untreated patients. Although there was no clear correlation between the change of rCMRGlc following the first course of iaACNU and the clinical outcome in this small group of patients, the patient with the most pronounced decrease of tumour metabolism (-16.5%) after the first course of iaACNU exhibited full tumour remission for 12 months, while the patient with the most pronounced increase of tumour metabolism (+65%) after the first course of iaACNU developed rapid tumour progression. The first results indicate that early effects of intra-arterial chemotherapy may be observed with 18FDG PET, especially following the first course of iACNU. Further studies are needed to evaluate the predictive value of such studies for therapy response.