The objective of this study was to evaluate the usefulness and impact of insulin administration before an 18F-FDG PET/computed tomography (CT) examination in diabetic patients in order to propose an optimized protocol that can reduce blood glucose levels without increasing muscular 18F-FDG uptake. A total of 130 patients underwent an 18F-FDG PET/CT. Twenty patients had glucose levels greater than 180 mg/dl and received endovenous insulin before 18F-FDG injection (group 1); 10 patients had glucose levels greater than 160 mg/dl and lower than 200 mg/dl and received no insulin (group 2); 100 patients were euglycemic (group 3). Biodistribution was adequate in 19 of 20 patients in group 1. Values of standardized uptake value in the gluteal muscle were 0.50±0.18 for group 1, 0.48±0.10 for group 2, and 0.49±0.08 for group 3; no significant differences in muscular 18F-FDG uptake could be found among the three groups. No adverse events were recorded. In conclusion, our protocol has been demonstrated to be safe and effective, with only a minor impact on glucose biodistribution and apparently without affecting PET accuracy.