OBJECTIVE. Our goal was to assess the sensitivity of positron emission tomography (PET) with 18F-fluorpdeoxyglucose (FDG) for the detection of mucinous carcinoma and to determine the histologic features of these tumors that may affect lesion detectability. MATERIALS AND METHODS. A retrospective review of all patients with mucinous carcinoma who had undergone FDG PET at our institution from 1995 through 1998 identified 25 patients with new or recurrent mucinous carcinoma at the time of PET. In 22 of these patients, tissue specimens available from either core biopsy or surgical resection allowed detailed histologic analysis. RESULTS. FDG PET revealed mucinous carcinoma in only 13 (59%) of 22 patients, resulting in an unusually high percentage (41%) of false-negative results. Two histologic features were found to be predictive of FDG PET results: tumor cellularity (p = 0.011) and the amount of mucin within the tumor mass (p = 0.009). There was a positive correlation between tumor FDG uptake and cellularity but a negative correlation with the amount of mucin. CONCLUSION. FDG PET is limited in the evaluation of mucinous tumors, particularly in hypocellular lesions with abundant mucin.