Purpose: This study investigated the uptake of [ ¹⁸F]2-fluoro-2- deoxy-glucose ([ ¹⁸F]FDG) in the human tumour xenograft FaDu at early time points after single dose irradiation with Positron-Emission-Tomography (PET), autoradiography and functional histology. Materials and methods: [ ¹⁸F]FDG-PET of FaDu hSCC xenografts on nude mice was performed before 25 Gy or 35 Gy single dose irradiation and one, seven or 11 days post irradiation (p.irr.). Before the second PET, mice were injected with pimonidazole (pimo) and bromodeoxyuridine (BrdU). After the PET tumours were excised, sliced and subjected to autoradiography and functional histology staining (pimo, BrdU, Ki67). [ ¹⁸F]FDG tumour uptake was quantified in the PET scans by maximal standard uptake value (SUVmax) and in the autoradiography after co-registration to the histology slices. Results: No differences in the overall [18F]FDG uptake between the two dose groups and time points were found with PET or autoradiography. Comparing autoradiography and histology, the [ ¹⁸F]FDG uptake was constant in tumour necrosis over time, while it decreased in vital tumour areas and particularly in hypoxic regions. No differences in the [ ¹⁸F]FDG uptake between positive and negative areas of Ki67 and BrdU were found. Conclusions: The decline of [ ¹⁸F]FDG uptake in vital tumour and in pimopositive areas as seen in autoradiography, was not reflected by evaluation of SUVmax determined by PET. These findings suggest that the SUVmax does not necessarily reflect changes in tumour biology after irradiation.