Eight patients with proven or suspected neoplastic lesions were examined with 1-aminocyclobutane C-11-carboxylic acid and 1-aminocyclopentane C-11-carboxylic acid using positron emission computed tomography. The results of this comparative study have shown that both of these unnatural amino acids have a high affinity for malignant tumors. The potential of nonmetabolizable C-11-labeled amino acids as noninvasive indicators of metabolic tumor activity is demonstrated and discussed.