Objective This radiotherapy planning study evaluated tumour delineation using both optimally respiratory gated and nongated fluorine-18 fluorodeoxyglucose-PET (18F-FDG-PET). Methods For 22 non-small-cell lung tumours, both scans were used to create the nongated and gated (g) gross tumour volumes (GTVg) together with the accompanying clinical target volumes (CTV) and planning target volumes (PTV). The size of the target volumes (TV) was evaluated and the accompanying radiotherapy plans were created to study the radiation doses to the organs at risk (OAR). Results The median volumes of GTVg, CTVg and PTVg were statistically significantly smaller compared with the corresponding nongated volumes, resulting in a median TV reduction of 0.5 cm3 (interquartile range 0.1-1.2), 1.5 cm3 (-0.2 to 7.0) and 2.3 cm3 (-0.5 to 11.3) for the GTVg, CTVg and PTVg, respectively. For the OAR, only the percentage of lung (GTV included) receiving at least 35 Gy was significantly smaller after gating, with a median difference in lung volume receiving at least 35 Gy of 5.7 cm3 (interquartile range -0.8 to 30.50). Conclusion Compared with nongated 18F-FDG-PET, the TVs obtained with optimally respiratory gated 18F-FDG-PET were significantly smaller, however, without a clinically relevant difference in radiation dose to the OAR.