Positron-emission tomography combined with X-ray computed tomography and using [ 18 F]-fluorodeoxyglucose (FDG-PET/CT) is being used for staging, restaging, followup, and treatment monitoring of several glucose avid tumors. Since FDG and glucose compete for the same transporters and for hexokinase, hyperglycemia may significantly modify the value of the semi-quantitative variables commonly used for estimating the uptake of FDG, including the standardized uptake value (SUV). Therefore, this issue is of particular relevance, when FDG-PET/CT is used in patients with poorly controlled diabetes mellitus. Thus, because of the competition between FDG and glucose, the control of glycemia may be relevant when imaging is being pursued, with the use of SUV, to estimate the degree of malignancy during the staging of the tumor and in particular for the comparison during follow-up and in case of therapy monitoring. Indeed this issue has been considered, since PET with FDG was in the early phase of introduction into clinical practice about 40 years ago. In spite of the acknowledgement of this very relevant issue, demonstrated by a simple PubMed search using the terms "FDG PET" and "blood glucose levels" and "FDG