The role for radiotherapy in patients with low-grade gliomas remains controversial. Two large prospective studies have failed to demonstrate a radiotherapeutic dose-response effect, and EORTC trial 22845 found no difference in survival between patients receiving adjuvant radiotherapy and those who received radiotherapy at tumour progression. The aim of this retrospective study was to analyse the patterns of carbon-11 methionine (MET) uptake on positron emission tomography (PET) in tumours treated with immediate radiotherapy and in those treated with delayed radiotherapy at the time of tumour progression. The 21 adult patients studied had histologically confirmed low-grade gliomas and had undergone a pre-treatment PET scan and a follow-up PET scan at the time of progression. Eleven of the patients had undergone initial radiotherapy a median of 5 weeks after the surgical procedure. The median time to progression was 3.5 years for this group, compared with 1.6 years for the group with delayed radiotherapy ( P=0.06). At the time of progression, non-irradiated tumours had a significantly higher MET uptake ( P=0.02) and a larger uptake volume ( P=0.008) compared with baseline, whereas irradiated tumours showed no statistically significant change. We observed a correlation between high pre-treatment uptake of MET and reduction in MET uptake in response to radiotherapy ( P=0.008). All irradiated tumours recurred within the radiation field. In conclusion, our results demonstrate signs of a residual radiation effect at the time of tumour progression in low-grade gliomas with high pre-treatment uptake of MET. Pre-treatment methionine uptake may be a marker for the radiosensitivity of low-grade gliomas.