Classification and treatment strategy of oligodendrogliomas (ODG) remain controversial. Imaging relies essentially on contrast enhancement using CT or MRI. The aim of our study was to use positron emission tomography (PET) using [18F]-flurodeoxyglucose (FDG) and [11C]-L-methyl-methionine (MET) to evaluate metabolic characteristics of ODG. We studied 19 patients with proven ODG, comparing standardized uptake values (SUV) and maximal tumor/contralateral normal tissues ratios (T/N). Imaging findings were compared with WHO, Smith and Daumas-Duport classifications. Uptake of FDG was decreased only in 8 patients, independently of grading, while MET uptake was always increased. MET uptake was significantly higher for high grade tumors grouped according to Smith or Daumas-Duport classifications, while no significant difference in MET uptake was found when using WHO classification. A different correlation was found between FDG and MET uptakes in normal tissues and high grade tumors. A trend for improved progression free survival was found for tumors that lacked contrast enhancement on MRI or those showing low FDG or MET uptake. In conclusion, MET appeared more sensitive than FDG to detect proliferation in ODG. The preferential protein metabolism, already noticeable for low-grade tumor, correlated with glucose metabolism and helped to separate, in vivo, high and low grade tumors.