A comparative study of tumor detection with ten 11C-labeled amino acids including four newly synthesized amino acids was carried out to find the most valuable 11C-labeled amino acid for the diagnosis of cancer. 11C-L-methionine showed the highest uptake by the experimental rat hepatoma AH109A (2.7% administered dose/g at 20 min, tumor to blood ratio; 11.4). The second highest uptake was of 11C-aminocyclopentane-carboxylic acid (ACPC). The newly synthesized 11C-DL-methyl-ACPC characteristically showed higher accumulation in tumor than in liver and the tumor to liver ratio reached 3.0 at 60 min after injection. It is suggested that 11C-L-methionine and 11C-DL-methyl-ACPC are useful amino acids for the diagnosis of cancer using positron emission tomography.