ABSTRACT: BACKGROUND: Using positron emission tomography (PET), we compared two tracers, [11C]metomidate ([11C]MTO) and [11C]acetate ([11C]ACE), for the characterization of hepatic tumors. METHODS: Thirty-three patients underwent PET with [11C]MTO and [11C]ACE and magnetic resonance imaging (MRI). Based on the histology of the tumor biopsy, 14 patients had hepatocellular carcinoma (HCC), 9 patients had focal nodular hyperplasia (FNH), and 10 patients had other types of hepatic tumors. Tumor uptake was evaluated by calculating the maximum and mean standardized uptake value and tumor-to-liver ratio. RESULTS: Altogether, 120 hepatic lesions (59 HCC, 18 FNH, 30 metastases of different primaries, 9 adenomas, and 4 regenerating nodules of liver cirrhosis) were detected by MRI. The overall tumor detection rate was slightly higher for [11C]MTO (39%) than for [11C]ACE (33%). [11C]ACE was more sensitive for HCC detection (50% versus 43%, respectively), whereas [11C]MTO was more sensitive for FNH detection (78% versus 44%, respectively). In HCC patients, the tumor grade correlated with [11C]ACE, but not with [11C]MTO. All of the patients with liver metastases, from various primary tumors (n = 10), were negative for both tracers. CONCLUSIONS: Due to low sensitivity, [11C]MTO and [11C]ACE PET have only limited value in diagnosing hepatic tumors.