(18)F-Fluorodeoxyglucose ((18)F-FDG) is the most common molecular imaging agent in oncology, with a high sensitivity and specificity for detecting several cancers. Antibodies could enhance specificity; therefore, procedures were developed for radiolabeling a small ( approximately 1451 Da) hapten peptide with (68)Ga or (18)F to compare their specificity with (18)F-FDG for detecting tumors using a pretargeting procedure. Mice were implanted with carcinoembryonic antigen (CEA; CEACAM5)-expressing LS174T human colonic tumors and a CEA-negative tumor, or an inflammation was induced in thigh muscle. A bispecific monoclonal anti-CEA x anti-hapten antibody was given to mice, and 16 hours later, 5 MBq of (68)Ga- or (18)F-labeled hapten peptides were administered intravenously. Within 1 hour, tissues showed high and specific targeting of (68)Ga-IMP-288, with 10.7 +/- 3.6% ID/g uptake in the tumor and very low uptake in normal tissues (e.g., tumor-to-blood ratio of 69.9 +/- 32.3), in a CEA-negative tumor (0.35 +/- 0.35% ID/g), and inflamed muscle (0.72 +/- 0.20% ID/g). (18)F-FDG localized efficiently in the tumor (7.42 +/- 0.20% ID/g) but also in the inflamed muscle (4.07 +/- 1.13% ID/g) and in several normal tissues; thus, pretargeted (68)Ga-IMP-288 provided better specificity and sensitivity. Positron emission tomography (PET)/computed tomography images reinforced the improved specificity of the pretargeting method. (18)F-labeled IMP-449 distributed similarly in the tumor and normal tissues as the (68)Ga-labeled IMP-288, indicating that either radiolabeled hapten peptide could be used. Thus, pretargeted immuno-PET does exceptionally well with short-lived radionuclides and is a highly sensitive procedure that is more specific than (18)F-FDG-PET. Mol Cancer Ther; 9(4); 1019-27. (c)2010 AACR.