In order to evaluate the methionine uptake of a glioma with positron emission tomography (PET), the kinetics of carbon-11 methionine was investigated in 11 patients by measuring the free 11C-methionine in plasma as an input function following intravenous administration. When the mean clearance curve of free 11C-methionine in plasma instead of individual 11C-methionine clearance curves was used, mean differences in uptake rate and distribution volume were 4.0% and 4.6% respectively. By applying the mean clearance curve of free 11C-methionine in 18 glioma patients, significant differences in 11C-methionine uptake rate and distribution volume were found according to pathological grading. For the accurate evaluation of the metabolism of 11C-methionine, it is therefore preferable that the actual level of free 11C-methionine in the plasma be measured, especially for the follow-up of individual cases. The study also demonstrated that the mean clearance curve of 11C-methionine in plasma might be employed as an input curve for calculating the uptake rate and distribution volume with small errors.