18F-fluoro-2-deoxy-D-glucose (FDG) positron emission tomography (PET) scan is used to evaluate various kinds of tumors. While most studies on PET findings of the colon focus on the colonic uptake pattern, studies regarding background colonic uptake on PET scan are rare. The purpose of this study was to identify the association between the background colonic uptake and the presence of colorectal adenoma (CRA), which is a frequent precancerous lesion. We retrospectively reviewed the medical records of 241 patients with gynecologic malignancy who had received PET or PET/computed tomography (CT) scan and colonoscopy at the same period as a baseline evaluation. Background colonic 18F-FDG uptake was visually graded and the maximal standardized uptake values (SUVmax ) of 7 different bowel segments were averaged. In univariate analysis, older age at diagnosis ( 50 years, p = 0.034), overweight (BMI ≥ 23 kg/m?, p = 0.010), hypercholesterolemia ( 200 mg/dL, p = 0.027), and high grade background colonic uptake (p = 0.009) were positively associated with the prevalence of CRA. By multiple logistic regression, high grade background colonic uptake was independently predictive of CRA (odds ratio = 2.25, p = 0.021). The proportion of CRA patients significantly increased as background colonic uptake grade increased from 1 to 4 (trend p = 0.015). Out of the 138 patients who underwent PET/CT, the proportion of CRA patients in the group with high SUVmax (> 2.25) was significantly higher than in the low SUVmax group (27.5% vs. 11.6%, p = 0.031). In conclusion, high grade of background colonic 18F-FDG uptake is significantly associated with the prevalence of CRA.