11C-methionine positron emission tomography (PET) and proliferating cell nuclear antigen (PCNA) staining were performed in 13 cases of glioma to investigate the relationship between the uptake of L-[methyl]-11C-methionine and the degree of malignancy and proliferative potential. The 11C-methionine uptake was significantly greater in high-grade gliomas compared to low-grade gliomas (P<0.05). The PCNA indexes were also significantly higher in the high-grade cases (P<0.05). Moreover, a strong positive correlation was found between the 11C-methionine values and the PCNA indexes (P<0.005), demonstrating that higher 11C-methionine uptake was associated with greater proliferative potential and greater malignancy. 11C-methionine PET is a potentially useful preoperative method to discriminate the malignancy of glioma.