The role of postoperative radiotherapy in patients with low grade gliomas is not established yet. PET with 11C methionine (MET) and 18F fluorodeoxyglucose (FDG) was used to perform cross sectional comparisons as well as within patient follow up studies in 30 operated patients with fibrillary astrocytoma WHO II. Uptake of tracer by tumour was quantified by radioactivity concentration ratios in tumour over contralateral brain (T/C). Comparing patients who did (n=13) or did not (n=17) receive external radiotherapy subsequent to first tumour resection, no differences in MET and FDG T/C between both groups were found during a postoperative period of 94 months (when recurrence and malignant progression of low grade astrocytomas are expected). Malignant progression occurred at a similar rate in both patient groups at a mean (SD) postoperative interval of 46 (26) months. Irrespective of whether radiotherapy was applied or not, malignant tumour recurrences showed higher T/C values (MET: 1.70 (0.64), FDG: 0.98 (0.23)) than recurrences without signs of malignancy (MET: 1.21 (0.21), FDG: 0.82 (0.08)) (Mann-Whitney: MET p=0.086, FDG p=0.035). The data show a relative lack of radiotherapy administered immediately after first tumour resection. In the course of disease, patients with tumours undergoing malignant progression may be identified with PET tracer methods.