11C-labeled L- and D-methionine uptake was measured in seven patients with brain tumors using positron emission tomography. Tumors accumulated both isomers of the tracer. The strongest uptake occurred in tumors with a high grade of malignancy, while low grade tumors accumulated less activity. The L to D uptake ratio in tumor regions ranged from 0.92-1.25. Conventional 99mTc-DTPA scans revealed blood-brain barrier damage in two patients with no or only slight 11C-methionine accumulation, while one patient with a negative 99mTc-DTPA scan accumulated 11C-methionine in the tumor region. In view of the biochemical pathway of methionine and the present findings, it is concluded that the uptake reflects metabolic activity in brain tissue rather than uptake by diffusion due to disruption of the blood-brain barrier.