This study aimed to test whether [(18)F]fluoro-D-glucose (FDG) uptake of tumours measured by positron emission tomography (PET) can be used as surrogate marker to define the optimal biological dose (OBD) of mTOR inhibitors in vivo. Everolimus at 0.05, 0.5, 5 and 15 mg kg(-1) per day was administered to gastric cancer xenograft-bearing mice for 23 days and FDG uptake of tumours was measured using PET from day 1 to day 8. To provide standard comparators for FDG uptake, tumour volume, S6 protein phosphorylation, Ki-67 staining and everolimus blood levels were evaluated. Everolimus blood levels increased in a dose-dependent manner but antitumour activity of everolimus reached a plateau at doses >or=5 mg kg(-1) per day (tumour volume treated vs control (T/C): 51% for 5 mg kg(-1) per day and 57% for 15 mg kg(-1) per day). Correspondingly, doses >or=5 mg kg(-1) per day led to a significant reduction in FDG uptake of tumours. Dose escalation above 5 mg kg(-1) per day did not reduce FDG uptake any further (FDG uptake T/C: 49% for 5 mg kg(-1) per day and 52% for 15 mg kg(-1) per day). Differences in S6 protein phosphorylation and Ki-67 index reflected tumour volume and changes in FDG uptake but did not reach statistical significance. In conclusion, FDG uptake might serve as a surrogate marker for dose finding studies for mTOR inhibitors in (pre)clinical trials.